[llvm] [PowerPC]: For shufflevector, don't commute when LHS and RHS are the same (PR #172443)

[Hongyu Chen via llvm-commits]

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XChy wrote:

What do you mean by "the codegen remained the same after I tried it"? It still crashes, or emits the same assembly?

https://github.com/llvm/llvm-project/pull/172443