[WebAssembly] Custom combines for f64x2.promote_low_f32x4

Replace the clang builtin function and LLVM intrinsic previously used to select
the f64x2.promote_low_f32x4 instruction with custom combines from standard
SelectionDAG nodes. Implement the new combines to share code with the similar
combines for f64x2.convert_low_i32x4_{s,u}. Resolves PR50232.

Differential Revision: https://reviews.llvm.org/D105675