[AMDGPU] Assembler: Fix VOP3 only instructions

Separate methods to convert parsed instructions to MCInst:

  - VOP3 only instructions (always create modifiers as operands in MCInst)
  - VOP2 instrunctions with modifiers (create modifiers as operands
    in MCInst when e64 encoding is forced or modifiers are parsed)
  - VOP2 instructions without modifiers (do not create modifiers
    as operands in MCInst)
  - Add VOP3Only flag. Pass HasMods flag to VOP3Common.
  - Simplify code that deals with modifiers (-1 is now same as
    0). This is no longer needed.
  - Add few tests (more will be added separately).
    Update error message now correct.

Patch By: Nikolay Haustov

Differential Revision: http://reviews.llvm.org/D16778

llvm-svn: 260483