TableGen: Add IsOptional field to AsmOperandClass

Summary:
This makes it possible to specify some operands as optional to the AsmMatcher.
Setting this field to true will prevent the AsmMatcher from emitting
'too few operands' errors when there are missing optional operands.

Reviewers: olista01, ab

Subscribers: nhaustov, arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D15755

llvm-svn: 259913