[WebAssembly] Add wasm_simd128.h intrinsics for relaxed SIMD

Add user-friendly intrinsic functions for all relaxed SIMD instructions
alongside the existing SIMD128 intrinsic functions in wasm_simd128.h. Test that
the new instrinsics lower to the expected instructions in the existing
cross-project-tests test file.

Reviewed By: aheejin, sbc100

Differential Revision: https://reviews.llvm.org/D150833