TableGen / AsmMatcher: Tweaks to avoid generating completely bogus match

functions.
 - Fix variant flattening when the variant embeds an operand reference.

 - Ignore instructions which reference an operand multiple times (e.g., "xorb
   $dst, $dst"), and operands which have extra flags (e.g., "$dst:subreg32").

llvm-svn: 78099