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

[Jay Foad via llvm-commits]

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@@ -127,14 +127,15 @@ body:             |
 
     %0 = IMPLICIT_DEF
     %1 = V_MOV_B32_e32 1078523331, implicit $exec
-    %2 = V_MAC_F32_e32 killed %0, %1, %3, implicit $mode, implicit $exec
+    %2 = IMPLICIT_DEF
+    %3 = V_MAC_F32_e32 killed %0, %1, %2, implicit $mode, implicit $exec
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jayfoad wrote:

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

I don't want to do that because changing the third operand in this case to be undef changes the behaviour of twoaddrs - it now tries to convert this MAC to a MAD, and the same for a bunch of other cases.

I could use a real instruction instead of IMPLICIT_DEF if that's preferable?

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