AMDGPU/GlobalISel: Add support for wide loads >= 256-bits

Summary:
This adds support for the most commonly used wide load types:
<8xi32>, <16xi32>, <4xi64>, and <8xi64>

Reviewers: arsenm

Reviewed By: arsenm

Subscribers: hiraditya, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, rovka, kristof.beyls, dstuttard, tpr, t-tye, volkan, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57399

llvm-svn: 365586