[AMDGPU] Assembler: refactor convert methods (VOP3 and MIMG)

Summary: Simplified converter methods for VOP3 and MIMG.

Reviewers: dp, artem.tamazov

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, tpr, vpykhtin, t-tye

Differential Revision: https://reviews.llvm.org/D35047

llvm-svn: 307407