[AMDGPU] Assembler: rework parsing of optional operands.

Summary:
Change process of parsing of optional operands. All optional operands use same parsing method - parseOptionalOperand().
No default values are added to OperandsVector.
Get rid of WORKAROUND_USE_DUMMY_OPERANDS_INSTEAD_MUTIPLE_DEFAULT_OPERANDS.

Reviewers: tstellarAMD, vpykhtin, artem.tamazov, nhaustov

Subscribers: arsenm, kzhuravl

Differential Revision: http://reviews.llvm.org/D20527

llvm-svn: 270556