[AMDGPU] Restrict v_cndmask_b32 abs/neg modifiers to f32

Summary:
D64497 allowed abs/neg source modifiers on v_cndmask_b32 but it doesn't
make any sense to apply them to f16 operands; they would interpret the
bits of the value as an f32, giving nonsensical results. This patch
restricts them to f32 operands.

Reviewers: arsenm, hakzsam

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64636

llvm-svn: 365904