[llvm] [AMDGPU] Fix register class constraints for si-fold-operands pass when folding immediate into copies (PR #131387)

[Matt Arsenault via llvm-commits]

================
@@ -1047,6 +1047,11 @@ void SIFoldOperandsImpl::foldOperand(
     if (MovOp == AMDGPU::COPY)
       return;
 
+    // Check if the destination register of the MOV operation belongs
+    // to a vector superclass. Folding would be illegal.
+    if (TRI->isVectorSuperClass(DestRC))
+      return;
----------------
arsenm wrote:

Would be better to query the register class of the result register, and check if there is a common class between it and the virtual register class. In the future it would be better to replace the instruction and modify the register class to enable folds in some cases 

https://github.com/llvm/llvm-project/pull/131387