[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;
       |