[WebAssembly] Remove saturating fp-to-int target intrinsics

Use the target-independent @llvm.fptosi and @llvm.fptoui intrinsics instead.
This includes removing the instrinsics for i32x4.trunc_sat_zero_f64x2_{s,u},
which are now represented in IR as a saturating truncation to a v2i32 followed by
a concatenation with a zero vector.

Differential Revision: https://reviews.llvm.org/D100596