Improve the AsmMatcher's ability to handle suboperands.

Improve the AsmMatcher's ability to handle suboperands.

When an operand class is defined with MIOperandInfo set to a list of
suboperands, the AsmMatcher has so far required that operand to also define
a custom ParserMatchClass, and InstAlias patterns have not been able to
set the individual suboperands separately.  This patch removes both of those
restrictions.  If a "compound" operand does not override the default
ParserMatchClass, then the AsmMatcher will now parse its suboperands
separately.  If an InstAlias operand has the same class as the corresponding
compound operand, then it will be handled as before; but if that check fails,
TableGen will now try to match up a sequence of InstAlias operands with the
corresponding suboperands.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@124314 91177308-0d34-0410-b5e6-96231b3b80d8