AMDGPU: Remove weird usage of implicit operand on COPY

For the purpose of the test it works as well to have a use after the
copy itself.

Author: arsenm

llvm/test/CodeGen/AMDGPU/coalescer-remat-dead-use.mir

@@ -93,6 +93,6 @@ body:             |
     %0:vgpr_32 = COPY $vgpr0
     %1:vgpr_32 = V_ADD_U32_e32 1, %0, implicit $exec
     %2:vgpr_32 = V_MOV_B32_e32 %1, implicit $exec
-    $vgpr0 = COPY killed %2, implicit %1
-    SI_RETURN_TO_EPILOG killed $vgpr0
+    $vgpr0 = COPY killed %2
+    SI_RETURN_TO_EPILOG killed $vgpr0, implicit %1
 ...