[llvm] [AMDGPU] Fix inline constant encoding for `v_pk_fmac_f16` (PR #176659)

[Matt Arsenault via llvm-commits]

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@@ -3662,6 +3695,29 @@ bool isPackedFP32Inst(unsigned Opc) {
   }
 }
 
+bool isPKFMACF16(unsigned Opc) {
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arsenm wrote:

Maintaining this is going to be terrible...Can you instead add a dedicated operand kind for this case? 

https://github.com/llvm/llvm-project/pull/176659