[InstCombine] Make indexed compare fold GEP source type independent (#71663)

The indexed compare fold converts comparisons of GEPs with same
(indirect) base into comparisons of offset. Currently, it only supports
GEPs with the same source element type.

This change makes the transform operate on offsets instead, which
removes the type dependence. To keep closer to the scope of the original
implementation, this keeps the limitation that we should only have at
most one variable index per GEP.

This addresses the main regression from
https://github.com/llvm/llvm-project/pull/68882.

TBH I have some doubts that this is really a useful transform (at least
for the case where there are extra pointer users, so we have to
rematerialize pointers at some point). I can only assume it exists for a
reason...