[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;
       |

llvm/lib/Target/AArch64/GISel/AArch64CallLowering.cpp

@@ -1022,7 +1022,7 @@ static unsigned getCallOpcode(const MachineFunction &CallerF, bool IsIndirect,
 
   if (!IsTailCall) {
     if (!PAI)
-      return IsIndirect ? getBLRCallOpcode(CallerF) : AArch64::BL;
+      return IsIndirect ? getBLRCallOpcode(CallerF) : (unsigned)AArch64::BL;
 
     assert(IsIndirect && "Direct call should not be authenticated");
     assert((PAI->Key == AArch64PACKey::IA || PAI->Key == AArch64PACKey::IB) &&