[WebAssembly] Fix names of SIMD instructions containing '_zero'

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.

Reviewed By: aheejin

Differential Revision: https://reviews.llvm.org/D121661