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

[Matt Arsenault via llvm-commits]

arsenm wrote:

I think this should be reverted and abandoned. This has nothing to do with SeparateConstOffsetFromGEP, and is another patch bolted at the front of it. The provided tests are non-canonical IR and I suspect this is just working around the depth limit issue in instcombine that @jrbyrnes was working on 

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