[AMDGPU] Assembler: Custom converters for SDWA instructions. Support for _dpp...

[AMDGPU] Assembler: Custom converters for SDWA instructions. Support for _dpp and _sdwa suffixes in mnemonics.

Summary:
Added custom converters for SDWA instruction to support optional operands and modifiers.
Support for _dpp and _sdwa suffixes that allows to force DPP or SDWA encoding for instructions.

Reviewers: tstellarAMD, vpykhtin, artem.tamazov

Subscribers: arsenm, kzhuravl

Differential Revision: http://reviews.llvm.org/D20625

llvm-svn: 271655