[SimplifyLibCalls] Remove invalid code comment in optimizeStringLength. NFC

[bjope]

Summary

The limitation in LibCallSimplifier::optimizeStringLength to only
optimize when the string is an i8 array was changed already in
commit 50ec0b5dceff1f3fe93 back in 2017.

We still only simplify when 's 'points at an array of 'CharSize', so
the comment is still valid in the sense that we do not support
arbitrary array types.

[nikic]

I believe this comment is about supporting arbitrary types, not just extension to wchar.

[bjope]

In D135261#3836686, @nikic wrote:

I believe this comment is about supporting arbitrary types, not just extension to wchar.

Alright. I see! So my confusion probably was that it says "i8" rather than "CharSize".

Before commit 50ec0b5dceff1f3fe93 the isGEPBasedOnPointerToString helper did limit this to i8 (but then we did not support CharSize!=8 here), so back then it was really limited to just i8. But nowadays I think the array need to match with "CharSize".
Still, as you say, we aren't supporting arbitrary types for the array elements (they need to match with "CharSize")!

[bjope]

Now just adjusting the code comment.

Description:

[SimplifyLibCalls] Adjust code comment in optimizeStringLength. NFC

The limitation in LibCallSimplifier::optimizeStringLength to only
optimize when the string is an i8 array was changed already in
commit 50ec0b5dceff1f3fe93 back in 2017.

We still only simplify when 's 'points at an array of 'CharSize', so
the comment is still valid in the sense that we do not support
arbitrary array types.

[nikic]

LG

llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp

Value *LibCallSimplifier::optimizeStringLength(CallInst *CI, IRBuilderBase &B,
   if (Bound)
     // Punt for strnlen for now.
     return nullptr;
 
   // If s is a constant pointer pointing to a string literal, we can fold
   // strlen(s + x) to strlen(s) - x, when x is known to be in the range
   // [0, strlen(s)] or the string has a single null terminator '\0' at the end.
   // We only try to simplify strlen when the pointer s points to an array
-  // of i8. Otherwise, we would need to scale the offset x before doing the
-  // subtraction. This will make the optimization more complex, and it's not
-  // very useful because calling strlen for a pointer of other types is
+  // of CharSize elements. Otherwise, we would need to scale the offset x before
+  // doing the subtraction. This will make the optimization more complex, and
+  // it's not very useful because calling strlen for a pointer of other types is
   // very uncommon.
   if (GEPOperator *GEP = dyn_cast<GEPOperator>(Src)) {
     // TODO: Handle subobjects.
     if (!isGEPBasedOnPointerToString(GEP, CharSize))
       return nullptr;
 
     ConstantDataArraySlice Slice;
     if (getConstantDataArrayInfo(GEP->getOperand(0), Slice, CharSize)) {