[WebAssembly] Implement remaining relaxed SIMD instructions

Add codegen, intrinsics, and builtins for the i16x8.relaxed_q15mulr_s,
i16x8.dot_i8x16_i7x16_s, and i32x4.dot_i8x16_i7x16_add_s instructions. These are
the last instructions from the relaxed SIMD proposal[1] that had not been
implemented.

[1]:
https://github.com/WebAssembly/relaxed-simd/blob/main/proposals/relaxed-simd/Overview.md.

Differential Revision: https://reviews.llvm.org/D127170