TableGen: AsmMatcher diagnostic when missing instruction mnemonic.
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: a4a6f32337ebd29ad4763b423022f00f68d1c7b7@156263 91177308-0d34-0410-b5e6-96231b3b80d8

lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx,lodyagin/bare_cxx