Silence C4715 warning from MSVC (NFC).

The warning started triggering after r317560.
This commit silences it in the same way as previously done in a similar
situation, see
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20140915/236088.html

llvm-svn: 317568

lib/CodeGen/GlobalISel/LegalizerInfo.cpp

@@ -307,6 +307,7 @@ LegalizerInfo::findAction(const SizeAndActionsVec &Vec, const uint32_t Size) {
   case NotFound:
     llvm_unreachable("NotFound");
   }
+  llvm_unreachable("Action has an unknown enum value");
 }
 
 std::pair<LegalizerInfo::LegalizeAction, LLT>