[AMDGPU] Rename pass "isel" to "amdgpu-isel"

Summary: The AMDGPU target specific pass "isel" is a misleading name.

Reviewers: tstellar, echristo, javed.absar, arsenm

Reviewed By: arsenm

Subscribers: arsenm, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D52759

llvm-svn: 343659