Mips: Silence a -Wcovered-switch-default

Remove a default label which covered no enumerators, replace it with a
llvm_unreachable.

No functionality changed.

llvm-svn: 212729
diff --git a/llvm/lib/Target/Mips/MCTargetDesc/MipsABIFlagsSection.cpp b/llvm/lib/Target/Mips/MCTargetDesc/MipsABIFlagsSection.cpp
index 3202b5e..52d5dd3 100644
--- a/llvm/lib/Target/Mips/MCTargetDesc/MipsABIFlagsSection.cpp
+++ b/llvm/lib/Target/Mips/MCTargetDesc/MipsABIFlagsSection.cpp
@@ -23,9 +23,9 @@
     if (Is32BitABI)
       return OddSPReg ? Val_GNU_MIPS_ABI_FP_64 : Val_GNU_MIPS_ABI_FP_64A;
     return Val_GNU_MIPS_ABI_FP_DOUBLE;
-  default:
-    return 0;
   }
+
+  llvm_unreachable("unexpected fp abi value");
 }
 
 StringRef MipsABIFlagsSection::getFpABIString(FpABIKind Value) {