[WebAssembly] memory.fill

Summary:
memset lowering, fix argument types in memcpy lowering, and
test encodings. Depends on D57736.

Reviewers: aheejin

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57791

llvm-svn: 353986