[CodeGen] Use MCRegister in RegisterMaskPair constructor. NFC (llvm#167274)

It's already used for the field in the struct.

Author: topperc

llvm/include/llvm/CodeGen/MachineBasicBlock.h

@@ -129,7 +129,7 @@ class MachineBasicBlock
     MCRegister PhysReg;
     LaneBitmask LaneMask;
 
-    RegisterMaskPair(MCPhysReg PhysReg, LaneBitmask LaneMask)
+    RegisterMaskPair(MCRegister PhysReg, LaneBitmask LaneMask)
         : PhysReg(PhysReg), LaneMask(LaneMask) {}
 
     bool operator==(const RegisterMaskPair &other) const {