[WebAssembly] Restore slashes in SIMD conversion names

Summary: Depends on D52372 and D52442.

Reviewers: aheejin, dschuff, aardappel

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

Differential Revision: https://reviews.llvm.org/D52512

llvm-svn: 343558