[llvm] [SeparateConstOffsetFromGEP] Decompose constant xor operand if possible (PR #150438)

[Jeffrey Byrnes via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
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jrbyrnes wrote:

Can you add some tests:

1. negative case -- where there is no disjoint bits.
2. variable operand of the xor is an operand with extractable offset (e.g. or-disjoint with a constant)
3. user of xor between the GEP where the user is an operand with an extractable offset
4. case where this disjoint bits for the xor constant produces an offset that is too large for the addressing mode.

https://github.com/llvm/llvm-project/pull/150438