[llvm] [AMDGPU] Prevent SDWA 'preserve' transformation for instructions in different basic blocks. (PR #82406)

[Jay Foad via llvm-commits]

jayfoad wrote:

> I probably cannot imagine situation when execmask at 'then' BB isn't a subset of execmasks from BBs for sdwa1 and sdwa2 instructions.

Right. We are in SSA form which guarantees that (for every thread) a definition of %v0 and %v1 dominates each use. After control flow lowering, this implies that the exec mask at the use will be a subset of the exec mask at the definitions.

https://github.com/llvm/llvm-project/pull/82406