[WebAssembly] Expand unavailable integer operations for vectors

Summary:
Expands for vector types all of the integer operations that are
expanded for scalars because they are not supported at all by
WebAssembly.

This CL has no tests because such tests would really be testing the
target-independent expansion, but I'm happy to add tests if reviewers
think it would be helpful.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D55010

llvm-svn: 347923