[AMDGPU] Lower llvm.amdgcn.s.buffer.load.v3[i|f]32

Summary: Add lowering support for 32-bit vec3 variant of s.buffer.load intrinsic.

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70118