[WebAssembly] Update relaxed-simd instruction names

Including builtin and intrinsic names. These should be the final names for the
proposal.
https://github.com/WebAssembly/relaxed-simd/blob/main/proposals/relaxed-simd/Overview.md

Reviewed By: aheejin, maratyszcza

Differential Revision: https://reviews.llvm.org/D138249