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

[Carl Ritson via llvm-commits]

perlfu wrote:

Can you explain the motivation for this a bit more?

I was under the impression that `LiveVariables` are meant to be going away.
Generally I don't think we use them (beyond maintaining them) in AMDGPU backend.

Also I would expect this to be called something like `SIFormSSAMemoryClauses` or `GCNFormSSAMemoryClauses`, i.e. maintaining an existing prefix with SSA somewhere later in the name.

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