[llvm] [DAGCombiner] Expose CSE for shifts of logic operations (PR #213091)

[Justin Fargnoli via llvm-commits]

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justinfargnoli wrote:

I'm okay with having a test that checks the `gep` expands to the pattern we're interested in, but I'd like to see the bulk of our tests check the actual pattern we're trying to match against `shl (zext (xor %base, Ci)), 2`.

https://github.com/llvm/llvm-project/pull/213091