[llvm] Reland "[Transforms] LoopIdiomRecognize recognize strlen and wcslen #108985" (PR #132572)

[Henry Jiang via llvm-commits]

================
@@ -295,7 +313,8 @@ bool LoopIdiomRecognize::runOnLoop(Loop *L) {
 
   // Disable loop idiom recognition if the function's name is a common idiom.
   StringRef Name = L->getHeader()->getParent()->getName();
-  if (Name == "memset" || Name == "memcpy")
+  if (Name == "memset" || Name == "memcpy" || Name == "strlen" ||
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mustartt wrote:

Yeah it does not work with mangled symbols. The original goal of this was to prevent the optimizer from calling the idiom function it self. If the idiom is in a namespace, it will still be transformed to
```asm
foo::strlen(char const*):
        jmp     strlen at PLT
``` 
which is arguably a lot better and more correct than accidentally creating an infinite tail call recursion:
```asm
strlen(char const*):
        jmp     strlen
``` 



https://github.com/llvm/llvm-project/pull/132572