[ConstantHoist] Remove check for notional overindexing

ConstantHoist currently only hoists GEPs if there is no notional
overindexing. As this transform only hoists address arithmetic,
it shouldn't care about whether any overindexing occurs or not.

There is one caveat: If the hoisted base GEP is inbounds, and a
later non-inbounds GEP is rewritten in terms of it, the value
may be incorrectly poisoned. To avoid this, restrict the transform
to inbounds GEPs for now, as the notional overindexing check
effectively did that as well. The inbounds restriction could be
dropped by dropping inbounds from the base GEP expression.

Differential Revision: https://reviews.llvm.org/D117201