[WebAssembly] SIMD conversions

Summary:
Lowers (s|u)itofp and fpto(s|u)i instructions for vectors. The fp to
int conversions produce poison values if their arguments are out of
the convertible range, so a future CL will have to add an LLVM
intrinsic to make the saturating behavior of this conversion usable.

Reviewers: aheejin, dschuff

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

Differential Revision: https://reviews.llvm.org/D52372

llvm-svn: 343052