TableGen: AsmMatcher diagnostic when missing instruction mnemonic.

Previously, if an instruction definition was missing the mnemonic,
the next line would just assert(). Issue a real diagnostic instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156263 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/utils/TableGen/AsmMatcherEmitter.cpp b/utils/TableGen/AsmMatcherEmitter.cpp
index 0cb86d0..96e882b 100644
--- a/utils/TableGen/AsmMatcherEmitter.cpp
+++ b/utils/TableGen/AsmMatcherEmitter.cpp
@@ -825,6 +825,9 @@
     throw TGError(TheDef->getLoc(),
                   "Instruction '" + TheDef->getName() + "' has no tokens");
   Mnemonic = AsmOperands[0].Token;
+  if (Mnemonic.empty())
+    throw TGError(TheDef->getLoc(),
+                  "Missing instruction mnemonic");
   // FIXME : Check and raise an error if it is a register.
   if (Mnemonic[0] == '$')
     throw TGError(TheDef->getLoc(),