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

gcc warned with
../lib/Target/ARM/ARMFrameLowering.cpp:797:31: warning: enumeral and non-enumeral type in conditional expression [-Wextra]
  797 |               Reg == ARM::R12 ? ARM::RA_AUTH_CODE : Reg, true);
      |               ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~