AMDGPU/GlobalISel: Implement select for G_EXTRACT

Reviewers: arsenm

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

Differential Revision: https://reviews.llvm.org/D49714

llvm-svn: 355156