[AMDGPU] SDWA: remove omod src operand for VOP2b instructions

Summary: VOP2b instructions (v_subbrev_u32, v_add_i32 ...) shouldn't support OMod operand in SDWA encoding

Reviewers: rampitec, dp

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

Differential Revision: https://reviews.llvm.org/D40172

llvm-svn: 318761