[CodeGen] Change MachineInstr::isConstantValuePHI to return Register. NFC.

llvm/include/llvm/CodeGen/MachineInstr.h

@@ -1764,8 +1764,8 @@ class MachineInstr
   bool isDereferenceableInvariantLoad() const;
 
   /// If the specified instruction is a PHI that always merges together the
-  /// same virtual register, return the register, otherwise return 0.
-  unsigned isConstantValuePHI() const;
+  /// same virtual register, return the register, otherwise return Register().
+  Register isConstantValuePHI() const;
 
   /// Return true if this instruction has side effects that are not modeled
   /// by mayLoad / mayStore, etc.

llvm/lib/CodeGen/MachineInstr.cpp

@@ -1535,19 +1535,16 @@ bool MachineInstr::isDereferenceableInvariantLoad() const {
   return true;
 }
 
-/// isConstantValuePHI - If the specified instruction is a PHI that always
-/// merges together the same virtual register, return the register, otherwise
-/// return 0.
-unsigned MachineInstr::isConstantValuePHI() const {
+Register MachineInstr::isConstantValuePHI() const {
   if (!isPHI())
-    return 0;
+    return {};
   assert(getNumOperands() >= 3 &&
          "It's illegal to have a PHI without source operands");
 
   Register Reg = getOperand(1).getReg();
   for (unsigned i = 3, e = getNumOperands(); i < e; i += 2)
     if (getOperand(i).getReg() != Reg)
-      return 0;
+      return {};
   return Reg;
 }
 

llvm/lib/CodeGen/MachineSSAUpdater.cpp

@@ -201,7 +201,7 @@ Register MachineSSAUpdater::GetValueInMiddleOfBlock(MachineBasicBlock *BB,
 
   // See if the PHI node can be merged to a single value.  This can happen in
   // loop cases when we get a PHI of itself and one other value.
-  if (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) {
+  if (Register ConstVal = InsertedPHI->isConstantValuePHI()) {
     InsertedPHI->eraseFromParent();
     return ConstVal;
   }