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

[Jay Foad via llvm-commits]

jayfoad wrote:

> It also adds some new ISel logic to make G_SELECT 1,0 be lowered to a simple SCC copy. This is because a copy of SCC is already a s_cselect x, 1, 0. Without that we get some regressions.

I don't understand why this part is required. Do you have a simple example?

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