[llvm] [AMDGPU] Merge two V_CNDMASK instructions into V_DUAL_CNDMASK (PR #135007)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I think it is OK to do it in SIShrinkInstructions. It does not have stricter rules than SIFoldOperands.

It kind of does, it needs to support SSA and non-SSA

https://github.com/llvm/llvm-project/pull/135007