[AMDGPU] Rename $dst operand to $vdst for VOP instructions.

Summary: This change renames output operand for VOP instructions from dst to vdst. This is needed to enable decoding named operands for disassembler.

Reviewers: vpykhtin, tstellarAMD, arsenm

Subscribers: arsenm, llvm-commits, nhaustov

Projects: #llvm-amdgpu-spb

Differential Revision: http://reviews.llvm.org/D16920

llvm-svn: 260986