[ConstantFolding] Canonicalize constexpr GEPs to i8 (#89872)

This patch canonicalizes constant expression GEPs to use i8 source
element type, aka ptradd. This is the ConstantFolding equivalent of the
InstCombine canonicalization introduced in #68882.

I believe all our optimizations working on constant expression GEPs
(like GlobalOpt etc) have already been switched to work on offsets, so I
don't expect any significant fallout from this change.

This is part of:
https://discourse.llvm.org/t/rfc-replacing-getelementptr-with-ptradd/68699