[llvm] [AMDGPU][GlobalISel] Expand SGPR S1 exts into G_SELECT (PR #68858)

[Pierre van Houtryve via llvm-commits]

Pierre-vh wrote:

Context: This is part of a series of patch to eliminate SGPR S1 from reaching GISel under most circumstances.
This is all preliminary work for a bigger patch series (by @petar-avramovic) which rewrite i1 PHI lowering for GISel.

https://github.com/llvm/llvm-project/pull/68858