[WebAssembly] Lower memmove to memory.copy

Summary: The lowering is identical to the memcpy lowering.

Reviewers: aheejin

Subscribers: dschuff, sbc100, jgravelle-google, hiraditya, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57727

llvm-svn: 353216