[ValueTracking][SimplifyLibCalls] Fix bug in getConstantDataArrayInfo for wchar_t

When SimplifyLibCalls is dealing with wchar_t (e.g. optimizing wcslen)
it uses ValueTracking helpers with a CharSize/ElementSize that isn't
8, but rather 16 or 32 (to match with the size in bits of a wchar_t).

Problem I've seen is that llvm::getConstantDataArrayInfo is taking
both an "ElementSize" argument (basically indicating size of a
char/element in bits) and an "Offset" which afaict is an offset
in the unit "number of elements". Then it also use
stripAndAccumulateConstantOffsets to get a "StartIdx" which afaict
is calculated in bytes. The returned Slice.Length is based on
arithmetics that add/subtract variables that are having different
units (bytes vs elements). Most notably I think the "StartIdx" must
be scaled using the "ElementSize" to get correct results.

The symptom of the above problem was seen in the wcslen-1.ll test
case which miscompiled.

This patch is supposed to resolve the bug by converting between
bytes and elements when needed.

Differential Revision: https://reviews.llvm.org/D135263