[AArch64] Fix gcc "enumeral and non-enumeral type" warning

Without this gcc (9.3.0) warns with
 ../lib/Target/AArch64/GISel/AArch64CallLowering.cpp: In function 'unsigned int getCallOpcode(const llvm::MachineFunction&, bool, bool, std::optional<llvm::CallLowering::PtrAuthInfo>&, llvm::MachineRegisterInfo&)':
 ../lib/Target/AArch64/GISel/AArch64CallLowering.cpp:1025: warning: enumeral and non-enumeral type in conditional expression [-Wextra]
  1025 |       return IsIndirect ? getBLRCallOpcode(CallerF) : AArch64::BL;
       |