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

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I tried doing this in InstCombine and the pass fail to reach a fixed point state and the pass is trying to merge back the two constants in to one. This is one of the reasons why I chose to do it SeparateConstOffset pass.
> 

Do you mean cases like the add 0, 0 from the tests? I'm confused about what this is trying to do . You should not be trying to create these. These also should not be in these tests (other than maybe one if you need to check a degenerate case doesn't crash)

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