[WebAssembly] Implement i64x2 comparisons

Removes the prototype builtin and intrinsic for i64x2.eq and implements that
instruction as well as the other i64x2 comparison instructions in the final SIMD
spec. Unsigned comparisons were not included in the final spec, so they still
need to be scalarized via a custom lowering.

Differential Revision: https://reviews.llvm.org/D99623