AMDGPU: Implement getMemcpyLoopLoweringType

Summary: Based on a patch by Matt Arsenault.

Reviewers: rampitec, kerbowa, nhaehnle, arsenm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77057