[llvm] [SeparateConstOffsetFromGEP] Decompose constant xor operand if possible (PR #135788)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Try to transform I = xor(A, C1) into or disjoint(Y, C2) where Y = xor(A, C0) is another existing instruction dominating I,
> C2 = C1 - C0, and A is known to be disjoint with C2.

This transform also requires C0 and C2 to be disjoint, right? So `C2 = C1 - C0` can also be written as `C2 = C1 ^ C0`.

https://github.com/llvm/llvm-project/pull/135788