[WebAssembly] Restore defaults for stores per memop

Summary:
Large slowdowns were observed in Rust due to many small, constant
sized copies in conjunction with poorly-optimized memory.copy
implementations. Since memory.copy cannot be expected to be inlined
efficiently by engines at this time, stop using it for the smallest
copies. We continue to lower all memcpy intrinsics to memory.copy,
though.

Reviewers: aheejin, alexcrichton

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67639

llvm-svn: 372275