[SystemZ] Fix build bot failures

My patch 9db13b5 seems to have
caused some build bots to fail due to warnings that appear only
when using -Wcovered-switch-default.

This patch is an attempt to fix this by trying to avoid both the warning
"default label in switch which covers all enumeration values"
for the inner switch statements and at the same time the warning
"this statement may fall through"
for the outer switch statement in getVectorComparison
(SystemZISelLowering.cpp).

Author: uweigand

llvm/lib/Target/SystemZ/SystemZISelLowering.cpp

@@ -2647,8 +2647,8 @@ static unsigned getVectorComparison(ISD::CondCode CC, CmpMode Mode) {
     case CmpMode::FP:          return SystemZISD::VFCMPE;
     case CmpMode::StrictFP:    return SystemZISD::STRICT_VFCMPE;
     case CmpMode::SignalingFP: return SystemZISD::STRICT_VFCMPES;
-    default:                   llvm_unreachable("Bad mode");
     }
+    llvm_unreachable("Bad mode");
 
   case ISD::SETOGE:
   case ISD::SETGE:
@@ -2657,8 +2657,8 @@ static unsigned getVectorComparison(ISD::CondCode CC, CmpMode Mode) {
     case CmpMode::FP:          return SystemZISD::VFCMPHE;
     case CmpMode::StrictFP:    return SystemZISD::STRICT_VFCMPHE;
     case CmpMode::SignalingFP: return SystemZISD::STRICT_VFCMPHES;
-    default:                   llvm_unreachable("Bad mode");
     }
+    llvm_unreachable("Bad mode");
 
   case ISD::SETOGT:
   case ISD::SETGT:
@@ -2667,17 +2667,17 @@ static unsigned getVectorComparison(ISD::CondCode CC, CmpMode Mode) {
     case CmpMode::FP:          return SystemZISD::VFCMPH;
     case CmpMode::StrictFP:    return SystemZISD::STRICT_VFCMPH;
     case CmpMode::SignalingFP: return SystemZISD::STRICT_VFCMPHS;
-    default:                   llvm_unreachable("Bad mode");
     }
+    llvm_unreachable("Bad mode");
 
   case ISD::SETUGT:
     switch (Mode) {
     case CmpMode::Int:         return SystemZISD::VICMPHL;
     case CmpMode::FP:          return 0;
     case CmpMode::StrictFP:    return 0;
     case CmpMode::SignalingFP: return 0;
-    default:                   llvm_unreachable("Bad mode");
     }
+    llvm_unreachable("Bad mode");
 
   default:
     return 0;