llvm-mc/AsmParser: Add hack to ignore Int_* and *_Int instructions for now, to

make it easier to see interesting ambiguities.
 - Also, check that user doesn't try to redefine the super class. This is a wart
   in the current design, in that assembler match classes aren't explicitly
   declared somewhere (so there isn't a unique place to declare the super
   class). This should probably be fixed.

llvm-svn: 78532