[AMDGPU] Tests for unnecessary S_WAIT_XCNT insertion (#145688)

Hardware does an implicit "S_WAIT_XCNT 0" between SMEM and VMEM
instructions, so there will never be outstanding address translations
for both SMEM and VMEM at the same time.

Author: jayfoad

llvm/test/CodeGen/AMDGPU/wait-xcnt.mir

@@ -966,3 +966,45 @@ body: |
     $vgpr2 = V_MOV_B32_e32 $vgpr2, implicit $exec
     $sgpr0 = S_MOV_B32 0
 ...
+
+# TODO: Unnecessary wait before overwriting vgpr0.
+---
+name: overwrite_vgpr_after_smem
+tracksRegLiveness: true
+machineFunctionInfo:
+  isEntryFunction: true
+body: |
+  bb.0:
+    liveins: $vgpr0_vgpr1, $sgpr0_sgpr1
+    ; GCN-LABEL: name: overwrite_vgpr_after_smem
+    ; GCN: liveins: $vgpr0_vgpr1, $sgpr0_sgpr1
+    ; GCN-NEXT: {{  $}}
+    ; GCN-NEXT: $vgpr2 = GLOBAL_LOAD_DWORD $vgpr0_vgpr1, 0, 0, implicit $exec
+    ; GCN-NEXT: $sgpr2 = S_LOAD_DWORD_IMM $sgpr0_sgpr1, 0, 0
+    ; GCN-NEXT: S_WAIT_XCNT 0
+    ; GCN-NEXT: $vgpr0 = V_MOV_B32_e32 0, implicit $exec
+    $vgpr2 = GLOBAL_LOAD_DWORD $vgpr0_vgpr1, 0, 0, implicit $exec
+    $sgpr2 = S_LOAD_DWORD_IMM $sgpr0_sgpr1, 0, 0
+    $vgpr0 = V_MOV_B32_e32 0, implicit $exec
+...
+
+# TODO: Unnecessary wait before overwriting sgpr0.
+---
+name: overwrite_sgpr_after_vmem
+tracksRegLiveness: true
+machineFunctionInfo:
+  isEntryFunction: true
+body: |
+  bb.0:
+    liveins: $vgpr0_vgpr1, $sgpr0_sgpr1
+    ; GCN-LABEL: name: overwrite_sgpr_after_vmem
+    ; GCN: liveins: $vgpr0_vgpr1, $sgpr0_sgpr1
+    ; GCN-NEXT: {{  $}}
+    ; GCN-NEXT: $sgpr2 = S_LOAD_DWORD_IMM $sgpr0_sgpr1, 0, 0
+    ; GCN-NEXT: $vgpr2 = GLOBAL_LOAD_DWORD $vgpr0_vgpr1, 0, 0, implicit $exec
+    ; GCN-NEXT: S_WAIT_XCNT 0
+    ; GCN-NEXT: $sgpr0 = S_MOV_B32 0
+    $sgpr2 = S_LOAD_DWORD_IMM $sgpr0_sgpr1, 0, 0
+    $vgpr2 = GLOBAL_LOAD_DWORD $vgpr0_vgpr1, 0, 0, implicit $exec
+    $sgpr0 = S_MOV_B32 0
+...