[AArch64] Fix gcc warning about mix of enumeral and non-enumeral types [NFC]

Change the return type of
 findScratchNonCalleeSaveRegister
to Register instead of unsigned.

Every place the function is called we already put the returned value in a
Register variable or compare it with another Register.

This fixes some gcc warnings:
 ../lib/Target/AArch64/AArch64FrameLowering.cpp:744: warning: enumeral and non-enumeral type in conditional expression [-Wextra]
   743 |     Register TargetReg = RealignmentPadding
       |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   744 |                              ? findScratchNonCalleeSaveRegister(&MBB)
       |                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   745 |                              : AArch64::SP;
       |
 ../lib/Target/AArch64/AArch64FrameLowering.cpp:803: warning: enumeral and non-enumeral type in conditional expression [-Wextra]
   802 |     Register ScratchReg = RealignmentPadding
       |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   803 |                               ? findScratchNonCalleeSaveRegister(&MBB)
       |                               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   804 |                               : AArch64::SP;
       |

Author: mikaelholmen

llvm/lib/Target/AArch64/AArch64FrameLowering.cpp

@@ -296,7 +296,7 @@ static int64_t getArgumentStackToRestore(MachineFunction &MF,
 static bool produceCompactUnwindFrame(MachineFunction &MF);
 static bool needsWinCFI(const MachineFunction &MF);
 static StackOffset getSVEStackSize(const MachineFunction &MF);
-static unsigned findScratchNonCalleeSaveRegister(MachineBasicBlock *MBB);
+static Register findScratchNonCalleeSaveRegister(MachineBasicBlock *MBB);
 
 /// Returns true if a homogeneous prolog or epilog code can be emitted
 /// for the size optimization. If possible, a frame helper call is injected.
@@ -1013,7 +1013,7 @@ static void getLiveRegsForEntryMBB(LivePhysRegs &LiveRegs,
 // but we would then have to make sure that we were in fact saving at least one
 // callee-save register in the prologue, which is additional complexity that
 // doesn't seem worth the benefit.
-static unsigned findScratchNonCalleeSaveRegister(MachineBasicBlock *MBB) {
+static Register findScratchNonCalleeSaveRegister(MachineBasicBlock *MBB) {
   MachineFunction *MF = MBB->getParent();
 
   // If MBB is an entry block, use X9 as the scratch register