[llvm] AMDGPU: Allow only VGPR wide sources in fp6/4/8 conversions (PR #127464)

[Pravin Jagtap via llvm-commits]

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@@ -1818,6 +1818,18 @@ class getVOP3VRegSrcForVT<ValueType VT, bit IsTrue16 = 0, bit IsFake16 = 0> {
          1                 : RegisterOperand<VGPR_32>);
 }
 
+// VGPR only VOP3 src with 9 bit encoding
+class getVOP3VSrcReg9ForVT<ValueType VT> {
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pravinjagtap wrote:

> > I am not sure whether updating `getVOP3VRegSrcForVT` to use VRegSrc_* which additionally uses 9 bit encoding for decoding src [...] is really required for all the uses of getVOP3VRegSrcForVT or not?
> 
> Can you try it and see if it works?

It does not work if :
1. `RegisterOperand` is used the new API `getVOP3VSrcReg9ForVT` instead of current VRegSrc_*
2.  `getVOP3VRegSrcForVT` is extended to use VRegSrc_* instead of `RegisterOperand` 

https://github.com/llvm/llvm-project/pull/127464