[llvm] [SimplifyCFG] Speculatively execute empty BBs with multiple predecessors (PR #120905)

[Antonio Frighetto via llvm-commits]

================
@@ -3506,6 +3506,96 @@ bool SimplifyCFGOpt::speculativelyExecuteBB(BranchInst *BI,
   return true;
 }
 
+/// Speculate a conditional basic block flattening the CFG.
+/// Compared to speculativelyExecuteBB, it allows \p ThenBB to have multiple
+/// predecessors other than the current BB. An illustration of this transform is
+/// turning this IR:
+/// \code
+///   BB:
+///     %cmp = icmp ult %x, %y
+///     br i1 %cmp, label %EndBB, label %ThenBB
+///   ThenBB:
+///     br label BB2
+///   EndBB:
+///     %phi = phi i1 [ true, %ThenBB ], [ false, %BB ], [ false, %OtherBB ]
+///     ...
+/// \endcode
+///
+/// Into this IR:
+/// \code
+///   BB:
+///     %cmp = icmp ult %x, %y
+///     %sel = select i1 %cmp, i1 true, i1 false
+///     br label %EndBB
+///   ThenBB:
+///     br label BB2
+///   EndBB:
+///     %phi = phi i1 [ %sel, %ThenBB ], [ false, %BB ], [ false, %OtherBB ]
----------------
antoniofrighetto wrote:

```suggestion
///     %phi = phi i1 [ true, %ThenBB ], [ %sel, %BB ], [ false, %OtherBB ]
```
Unless you meant to set ThenBB too?

https://github.com/llvm/llvm-project/pull/120905