[InstSimplify] Make sure offsets have same size in computePointerICmp()

The way this is currently implemented the accumulated offsets can
end up having a different size, which causes unnecessary
complication for further extension of the code.

Don't strip pointer casts at the start and rely on
stripAndAccumulate to do any necessary stripping. It gracefully
handles different index sizes and will always retain the width of
the original pointer index type.

This is not NFC, but unlikely to make any practical difference.