[AMDGPU] Enable SEXT divergence driven selection.

Summary: This change enable the divergence driven selection for the SEXT DAG opcode.

Reviewers: vpykhtin, rampitec

Reviewed By: vpykhtin

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

Differential Revision: https://reviews.llvm.org/D76230