[AMDGPU] Fix for "v_div_scale_f64 reg, vcc, ..." parsing

Summary:
Added support for "VOP3Only" attribute in VOP3bInst encoding.
Set VOP3Only=1 for V_DIV_SCALE_F64/32 insns.
Added support for multi-dest instructions in AMDGPUAs::cvt*().
Added lit test for "V_DIV_SCALE_F64|F32 vreg,vcc|sreg,vreg,vreg,vreg".

Reviewers: tstellarAMD, arsenm

Subscribers: arsenm, SamWot, nhaustov, vpykhtin

Differential Revision: http://reviews.llvm.org/D16995

Patch By: Artem Tamazov

llvm-svn: 260560