[ms-inline asm] Make sure we fail gracefully on parse errors. Parse errors

[ms-inline asm] Make sure we fail gracefully on parse errors.  Parse errors
should only occur on invalid input.  Instruction matching errors aren't
unexpected, so we can't rely on the AsmParsers HadError variable directly.
rdar://12840278

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@170037 91177308-0d34-0410-b5e6-96231b3b80d8