[AMDGPU] Select s_cselect

Summary:
Add patterns to select s_cselect in the isel.

Handle more cases of implicit SCC accesses in si-fix-sgpr-copies
to allow new patterns to work.

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

Tags: #llvm

Re-commit D81925 with a bugfix D82370.

Differential Revision: https://reviews.llvm.org/D81925
Differential Revision: https://reviews.llvm.org/D82370