[llvm] [NFC] Add `const` for readability (PR #116611)

[Piotr Fusik via llvm-commits]

================
@@ -581,30 +581,30 @@ class InnerLoopVectorizer {
   virtual void printDebugTracesAtEnd() {}
 
   /// The original loop.
-  Loop *OrigLoop;
+  Loop *const OrigLoop;
 
   /// A wrapper around ScalarEvolution used to add runtime SCEV checks. Applies
   /// dynamic knowledge to simplify SCEV expressions and converts them to a
   /// more usable form.
   PredicatedScalarEvolution &PSE;
 
   /// Loop Info.
-  LoopInfo *LI;
+  LoopInfo *const LI;
----------------
pfusik wrote:

My change says "`LI` cannot be mutated".
Your change would say "`*LI` cannot be mutated" and it does not compile:

```diff
diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index ca89d7676677..d4cf5046b722 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -589,7 +589,7 @@ protected:
   PredicatedScalarEvolution &PSE;

   /// Loop Info.
-  LoopInfo *const LI;
+  const LoopInfo *LI;

   /// Dominator Tree.
   DominatorTree *const DT;
```

```
/home/p.fusik/upstream/llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp: In member function ‘void llvm::InnerLoopVectorizer::emitIterationCountCheck(llvm::BasicBlock*)’:
/home/p.fusik/upstream/llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:2498:67: error: invalid conversion from ‘const llvm::LoopInfo*’ to ‘llvm::LoopInfo*’ [-fpermissive]
 2498 |       SplitBlock(TCCheckBlock, TCCheckBlock->getTerminator(), DT, LI, nullptr,
      |                                                                   ^~
      |                                                                   |
      |                                                                   const llvm::LoopInfo*
In file included from /home/p.fusik/upstream/llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:135:
/home/p.fusik/upstream/llvm-project/llvm/include/llvm/Transforms/Utils/BasicBlockUtils.h:288:34: note:   initializing argument 4 of ‘llvm::BasicBlock* llvm::SplitBlock(llvm::BasicBlock*, llvm::Instruction*, llvm::DominatorTree*, llvm::LoopInfo*, llvm::MemorySSAUpdater*, const llvm::Twine&, bool)’
  288 |                        LoopInfo *LI = nullptr,
      |                        ~~~~~~~~~~^~~~~~~~~~~~
/home/p.fusik/upstream/llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp: In member function ‘void llvm::InnerLoopVectorizer::createVectorLoopSkeleton(llvm::StringRef)’:
/home/p.fusik/upstream/llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:2577:18: error: invalid conversion from ‘const llvm::LoopInfo*’ to ‘llvm::LoopInfo*’ [-fpermissive]
 2577 |                  LI, nullptr, Twine(Prefix) + "middle.block");
      |                  ^~
      |                  |
      |                  const llvm::LoopInfo*
In file included from /home/p.fusik/upstream/llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:135:
/home/p.fusik/upstream/llvm-project/llvm/include/llvm/Transforms/Utils/BasicBlockUtils.h:288:34: note:   initializing argument 4 of ‘llvm::BasicBlock* llvm::SplitBlock(llvm::BasicBlock*, llvm::Instruction*, llvm::DominatorTree*, llvm::LoopInfo*, llvm::MemorySSAUpdater*, const llvm::Twine&, bool)’
  288 |                        LoopInfo *LI = nullptr,
      |                        ~~~~~~~~~~^~~~~~~~~~~~
/home/p.fusik/upstream/llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:2579:73: error: invalid conversion from ‘const llvm::LoopInfo*’ to ‘llvm::LoopInfo*’ [-fpermissive]
 2579 |       SplitBlock(LoopMiddleBlock, LoopMiddleBlock->getTerminator(), DT, LI,
      |                                                                         ^~
      |                                                                         |
      |                                                                         const llvm::LoopInfo*
In file included from /home/p.fusik/upstream/llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:135:
/home/p.fusik/upstream/llvm-project/llvm/include/llvm/Transforms/Utils/BasicBlockUtils.h:288:34: note:   initializing argument 4 of ‘llvm::BasicBlock* llvm::SplitBlock(llvm::BasicBlock*, llvm::Instruction*, llvm::DominatorTree*, llvm::LoopInfo*, llvm::MemorySSAUpdater*, const llvm::Twine&, bool)’
  288 |                        LoopInfo *LI = nullptr,
      |                        ~~~~~~~~~~^~~~~~~~~~~~
/home/p.fusik/upstream/llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp: In member function ‘llvm::BasicBlock* llvm::EpilogueVectorizerMainLoop::emitIterationCountCheck(llvm::BasicBlock*, bool)’:
/home/p.fusik/upstream/llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:7882:40: error: invalid conversion from ‘const llvm::LoopInfo*’ to ‘llvm::LoopInfo*’ [-fpermissive]
 7882 |                                    DT, LI, nullptr, "vector.ph");
      |                                        ^~
      |                                        |
      |                                        const llvm::LoopInfo*
In file included from /home/p.fusik/upstream/llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:135:
/home/p.fusik/upstream/llvm-project/llvm/include/llvm/Transforms/Utils/BasicBlockUtils.h:288:34: note:   initializing argument 4 of ‘llvm::BasicBlock* llvm::SplitBlock(llvm::BasicBlock*, llvm::Instruction*, llvm::DominatorTree*, llvm::LoopInfo*, llvm::MemorySSAUpdater*, const llvm::Twine&, bool)’
  288 |                        LoopInfo *LI = nullptr,
      |                        ~~~~~~~~~~^~~~~~~~~~~~
/home/p.fusik/upstream/llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp: In member function ‘virtual std::pair<llvm::BasicBlock*, llvm::Value*> llvm::EpilogueVectorizerEpilogueLoop::createEpilogueVectorizedLoopSkeleton(const SCEV2ValueTy&)’:
/home/p.fusik/upstream/llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:7923:73: error: invalid conversion from ‘const llvm::LoopInfo*’ to ‘llvm::LoopInfo*’ [-fpermissive]
 7923 |       SplitBlock(LoopVectorPreHeader, LoopVectorPreHeader->begin(), DT, LI,
      |                                                                         ^~
      |                                                                         |
      |                                                                         const llvm::LoopInfo*
In file included from /home/p.fusik/upstream/llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:135:
/home/p.fusik/upstream/llvm-project/llvm/include/llvm/Transforms/Utils/BasicBlockUtils.h:284:34: note:   initializing argument 4 of ‘llvm::BasicBlock* llvm::SplitBlock(llvm::BasicBlock*, llvm::BasicBlock::iterator, llvm::DominatorTree*, llvm::LoopInfo*, llvm::MemorySSAUpdater*, const llvm::Twine&, bool)’
  284 |                        LoopInfo *LI = nullptr,
      |                        ~~~~~~~~~~^~~~~~~~~~~~
```


https://github.com/llvm/llvm-project/pull/116611