[AMDGPU] Avoid constraining RC based on folded into operand (NFC) (#160743)

The RC of the folded operand does not need to be constrained based on
the RC of the current operand we are folding into.

The purpose of this PR is to facilitate this PR:
#151033

Author: JoshHuttonCode

llvm/lib/Target/AMDGPU/SIFoldOperands.cpp

@@ -712,10 +712,15 @@ bool SIFoldOperandsImpl::updateOperand(FoldCandidate &Fold) const {
           TII->getRegClass(MI->getDesc(), Fold.UseOpNo, TRI)) {
     const TargetRegisterClass *NewRC =
         TRI->getRegClassForReg(*MRI, New->getReg());
-    const TargetRegisterClass *ConstrainRC =
-        TRI->findCommonRegClass(OpRC, Old.getSubReg(), NewRC, New->getSubReg());
-    if (!ConstrainRC)
-      return false;
+
+    const TargetRegisterClass *ConstrainRC = OpRC;
+    if (New->getSubReg()) {
+      ConstrainRC =
+          TRI->getMatchingSuperRegClass(NewRC, OpRC, New->getSubReg());
+
+      if (!ConstrainRC)
+        return false;
+    }
 
     if (!MRI->constrainRegClass(New->getReg(), ConstrainRC)) {
       LLVM_DEBUG(dbgs() << "Cannot constrain " << printReg(New->getReg(), TRI)