[AMDGPU] AsmParser: Support for sext() modifier in SDWA. Some code cleaning in AMDGPUOperand.

Summary:
sext() modifier is supported in SDWA instructions only for integer operands. Spec is unclear should integer operands support abs and neg modifiers with sext - for now they are not supported.
Renamed InputModsWithNoDefault to FloatInputMods. Added SextInputMods for operands that support sext() modifier.
Added AMDGPUOperand::Modifier struct to handle register and immediate modifiers.
Code cleaning in AMDGPUOperand class: organize method in groups (render-, predicate-methods...).

Reviewers: vpykhtin, artem.tamazov, tstellarAMD

Subscribers: arsenm, kzhuravl

Differential Revision: http://reviews.llvm.org/D20968

llvm-svn: 272384