[ConstantFold] Remove notional over-indexing fold (#93697)

The data-layout independent constant folding currently has some rather
gnarly code for canonicalizing GEP indices to reduce "notional
overindexing", and then infers inbounds based on that canonicalization.

Now that we canonicalize to i8 GEPs, this canonicalization is
essentially useless, as we'll discard it as soon as the GEP hits the
data-layout aware constant folder anyway. As such, I'd like to remove
this code entirely.

This shouldn't have any impact on optimization capabilities.