[AMDGPU] Enable code selection using `s_mul_hi_u32`/`s_mul_hi_i32`.

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59501

llvm-svn: 356405