[ConstantFold] Don't convert getelementptr to ptrtoint+inttoptr

ConstantFolding currently converts "getelementptr i8, Ptr, (sub 0, V)"
to "inttoptr (sub (ptrtoint Ptr), V)". This transform is, taken by
itself, correct, but does came with two issues:

1. It unnecessarily broadens provenance by introducing an inttoptr.
   We generally prefer not to introduce inttoptr during optimization.
2. For the case where V == ptrtoint Ptr, this folds to inttoptr 0,
   which further folds to null. In that case provenance becomes
   incorrect. This has been observed as a real-world miscompile with
   rustc.

We should probably address that incorrect inttoptr 0 fold at some
point, but in either case we should also drop this inttoptr-introducing
fold. Instead, replace it with a fold rooted at
ptrtoint(getelementptr), which seems to cover the original
motivation for this fold (test2 in the changed file).

Differential Revision: https://reviews.llvm.org/D124677