Add tests checking that each SIMD intrinsic produces the expected instruction.
Assembly tests are generally discouraged in clang, but in this case we actually
care about the specific instruction being generated from the intrinsics. There
are nine problems with the current intrinsic codegen and they are marked in the
tests with FIXMEs.
Also fix the names of a few instructions to match the spec, fix the ordering of
*_const intrinsics, and add the missing wasm_i64x2_make intrinsic.