[llvm] [AMDGPU] Fix some MIR tests (PR #66090)

[Matt Arsenault via llvm-commits]

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@@ -185,7 +188,8 @@ body:             |
   bb.0:
 
     %0 = V_MOV_B32_e32 1078523331, implicit $exec
-    %1 = V_MAC_F32_e32 %stack.0, %0, %2, implicit $mode, implicit $exec
+    %1 = IMPLICIT_DEF
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arsenm wrote:

IMPLICIT_DEF isn't really representative of running twoaddrs, which runs after ProcessImplicitDefs. Better to have an undef operand 

https://github.com/llvm/llvm-project/pull/66090