[AMDGPU] Assembler: Support DPP instructions.

Supprot DPP syntax as used in SP3 (except several operands syntax).
Added dpp-specific operands in td-files.
Added DPP flag to TSFlags to determine if instruction is dpp in InstPrinter.
Support for VOP2 DPP instructions in td-files.
Some tests for DPP instructions.

ToDo:
  - VOP2bInst:
    - vcc is considered as operand
    - AsmMatcher doesn't apply mnemonic aliases when parsing operands
  - v_mac_f32
  - v_nop
  - disable instructions with 64-bit operands
  - change dpp_ctrl assembler representation to conform sp3

Review: http://reviews.llvm.org/D17804
llvm-svn: 263008