[WebAssembly] Expand operations not supported by SIMD

Summary:
This prevents crashes in instruction selection when these operations
are used. The tests check that the scalar version of the instruction
is used where applicable, although some expansions do not use the
scalar version.

Reviewers: aheejin

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58859

llvm-svn: 355261