special case the mnemonic operand of the instruction in the

generated matcher, emiting it as a column in the MatchEntry
table instead of forcing it to go through classification and
everything else.  Making it be classified caused tblgen to
produce a ton of one-off classes for each mneumonic.  This
should reduce the size of the generated matcher significantly
while paving the way for future improvements.

llvm-svn: 113169