[AMDGPU] Add pre-commit test for optimized KILL insertion.

[arsenm]

Test name could be a bit more specific

llvm/test/CodeGen/AMDGPU/optimize-kill.mir

+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
+# RUN: llc -mtriple=amdgcn-amd-amdhsa -mcpu=gfx900 -start-before=greedy,1 -stop-after=virtregrewriter,1 -verify-machineinstrs %s -o - | FileCheck -check-prefix=GCN %s
+
+# The virtregrewriter pass should not insert a KILL when found an identical copy with an implicit reserved register.
+# For the following instruction,
+#      %4:vgpr_32 = COPY %3:vgpr_32, implicit $exec
+# if the register allocated for both the source and destination operands remain the same (vgpr0, for instance), there is
+# no need to insert the KILL instruction as the implicit operand (exec) is a reserved register.
+#     $vgpr0 = KILL killed renamable $vgpr0, implicit $exec
+---
+name:            test
+tracksRegLiveness: true
+machineFunctionInfo:
+  isEntryFunction: true
+  stackPtrOffsetReg: '$sgpr32'
+body:             |
+  bb.0:
+    liveins: $sgpr0, $sgpr1
+    ; GCN-LABEL: name: test
+    ; GCN: liveins: $sgpr0, $sgpr1
+    ; GCN-NEXT: {{  $}}
+    ; GCN-NEXT: renamable $sgpr2_sgpr3 = COPY $exec
+    ; GCN-NEXT: $exec = S_WQM_B64 $exec, implicit-def $scc
+    ; GCN-NEXT: renamable $vgpr0 = COPY renamable $sgpr0
+    ; GCN-NEXT: renamable $vgpr0 = BUFFER_LOAD_DWORD_IDXEN killed renamable $vgpr0, undef renamable $sgpr0_sgpr1_sgpr2_sgpr3, 0, 0, 0, 0, implicit $exec
+    ; GCN-NEXT: renamable $vgpr0 = KILL killed renamable $vgpr0, implicit $exec
+    ; GCN-NEXT: $exec = S_AND_B64 $exec, killed renamable $sgpr2_sgpr3, implicit-def $scc
+    ; GCN-NEXT: SI_RETURN_TO_EPILOG $vgpr0
+    renamable $sgpr2_sgpr3 = COPY $exec
+    $exec = S_WQM_B64 $exec, implicit-def $scc
+    %2:vgpr_32 = COPY killed renamable $sgpr0
+    %3:vgpr_32 = BUFFER_LOAD_DWORD_IDXEN %2, undef renamable $sgpr0_sgpr1_sgpr2_sgpr3, 0, 0, 0, 0, implicit $exec
+    %4:vgpr_32 = COPY %3:vgpr_32, implicit $exec
+    $exec = S_AND_B64 $exec, killed renamable $sgpr2_sgpr3, implicit-def $scc
+    $vgpr0 = COPY %4
+    SI_RETURN_TO_EPILOG killed $vgpr0
+...