[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 50ec0b5d 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.

Differential Revision: https://reviews.llvm.org/D135261