[WebAssembly][SIMD] Rename shuffle, swizzle, and load_splats

These instructions previously used prefixes like v8x16 to signify that they were
agnostic between float and int interpretations. We renamed these instructions to
remove this form of prefix in https://github.com/WebAssembly/simd/issues/297 and
https://github.com/WebAssembly/simd/issues/316 and this commit brings the names
in LLVM up to date.

Differential Revision: https://reviews.llvm.org/D93722