[AMDGPU] Allow abs/neg source modifiers on v_cndmask_b32

Summary:
D59191 added support for these modifiers in the assembler and
disassembler. This patch just teaches instruction selection that it can
use them.

Reviewers: arsenm, tstellar

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64497

llvm-svn: 365640