[llvm] [AMDGPU] Add SSA-form memory clause pass (SSASIFormMemoryClauses) (PR #209656)

[Carl Ritson via llvm-commits]

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perlfu wrote:

LiveIntervals can exist prior to PHI Elimination.
I put in a few change in the past to ensure they were preserved when generated earlier, primarily to facilitate #161054.

https://github.com/llvm/llvm-project/pull/209656