[AMDGPU] Extend the SI Load/Store optimizer

Summary:
Extend the SI Load/Store optimizer to merge MIMG load instructions. Handle
different flavours of image_load and image_sample instructions.

When the instructions of the same subclass differ only in dmask, merge
them and update dmask accordingly.

Reviewers: nhaehnle

Reviewed By: nhaehnle

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64911

llvm-svn: 374984