[llvm] [AMDGPU] Mark S_NOP as having side effects (PR #65745)

[Jay Foad via llvm-commits]

================
@@ -63,9 +63,9 @@ body:             |
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT: bb.1:
   ; CHECK-NEXT:   S_NOP 0, implicit %1.sub2
+  ; CHECK-NEXT:   S_NOP 0, implicit undef %4.sub0:vreg_128
----------------
jayfoad wrote:

S_NOPs can't be reordered now, since they all have side effects.

https://github.com/llvm/llvm-project/pull/65745