AMDGPU/GlobalISel: Mark G_BITCAST s32 <--> <2 x s16> legal

Reviewers: arsenm

Reviewed By: arsenm

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

Differential Revision: https://reviews.llvm.org/D34129

llvm-svn: 305692