[ms-inline asm] Remove the MatchInstruction() function. Previously, this was

the interface between the front-end and the MC layer when parsing inline
assembly.  Unfortunately, this is too deep into the parsing stack. Specifically,
we're unable to handle target-independent assembly (i.e., assembly directives,
labels, etc.).  Note the MatchAndEmitInstruction() isn't the correct
abstraction either.  I'll be exposing target-independent hooks shortly, so this
is really just a cleanup.

llvm-svn: 165858