[gicombiner] Import tryCombineIndexedLoadStore()

Summary:
Now that arbitrary data is supported, import tryCombineIndexedLoadStore()

Depends on D69147

Reviewers: bogner, volkan

Reviewed By: volkan

Subscribers: hiraditya, arphaman, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D69151