[llvm] [AMDGPU][MC] Improve error message for missing dim operand (PR #96588)

[Matt Arsenault via llvm-commits]

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@@ -1089,7 +1089,7 @@ def exp_vm : NamedBitOperand<"vm", "ExpVM">;
 def FORMAT : CustomOperand<i8>;
 
 def DMask : NamedIntOperand<i16, "dmask">;
-def Dim : CustomOperand<i8>;
+def Dim : CustomOperand<i8, 1>;
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arsenm wrote:

Can you comment the parameter name here? I have to look to see what this does 

https://github.com/llvm/llvm-project/pull/96588