Fix the instruction names to match the WebAssembly spec:
- i32x4.trunc_sat_zero_f64x2_{s,u} => i32x4.trunc_sat_f64x2_{s,u}_zero
- f32x4.demote_zero_f64x2 => f32x4.demote_f64x2_zero
Also rename related things like intrinsics, builtins, and test functions to
match.