[InstSimplify] Strip offsets once in computePointerICmp()

Instead of doing an inbounds strip first and another non-inbounds
strip afterward for equality comparisons, directly do a single
inbounds or non-inbounds strip based on whether we have an equality
predicate or not.

This is NFC-ish in that the alloca equality codepath is the only
part that sees additional non-inbounds offsets now, and for that
codepath it doesn't matter whether or not the GEP is inbounds, as
it does a stronger check itself. InstCombine would infer inbounds
for such GEPs.

(cherry picked from commit f35af775)