[TableGen] AsmMatcher: support for default values for optional operands

Summary:
This change allows to specify "DefaultMethod" for optional operand (IsOptional = 1) in AsmOperandClass that return default value for operand. This is used in convertToMCInst to set default values in MCInst.
Previously if you wanted to set default value for operand you had to create custom converter method. With this change it is possible to use standard converters even when optional operands presented.

Reviewers: tstellarAMD, ab, craig.topper

Subscribers: jyknight, dsanders, arsenm, nhaustov, llvm-commits

Differential Revision: http://reviews.llvm.org/D18242

llvm-svn: 268726