[WebAssembly] Explicitly add {z,s}ext so extends are selected

During DAG legalization, {u,s}itofp instructions on v2i8, v2i16, v4i8
and v4i16 types ended up being legalized into scalar instructions, when
they could just be extended to v2i32/v4i32 instead.

Fixes https://github.com/llvm/llvm-project/issues/57182

Differential Revision: https://reviews.llvm.org/D140916