[llvm] [AMDGPU] Add regbankselect rules for G_ADD/SUB and variants (PR #159860)

[Anshil Gandhi via llvm-commits]

================
@@ -76,7 +76,6 @@ body: |
     ; CHECK-NEXT: [[C1:%[0-9]+]]:sgpr(s32) = G_CONSTANT i32 -1
     ; CHECK-NEXT: [[C2:%[0-9]+]]:sgpr(s32) = G_CONSTANT i32 0
     ; CHECK-NEXT: [[SELECT:%[0-9]+]]:sgpr(s32) = G_SELECT [[AND]](s32), [[C1]], [[C2]]
-    ; CHECK-NEXT: [[TRUNC:%[0-9]+]]:sgpr(s16) = G_TRUNC [[SELECT]](s32)
----------------
gandhi56 wrote:

I was thinking about your suggestion to add a use of %3 for sgpr s16. Why is the elimination of G_TRUNC an issue?

https://github.com/llvm/llvm-project/pull/159860