[WebAssembly] Codegen for f64x2.convert_low_i32x4_{s,u}

Add a custom DAG combine and ISD opcode for detecting patterns like

  (uint_to_fp (extract_subvector ...))

before the extract_subvector is expanded to ensure that they will ultimately
lower to f64x2.convert_low_i32x4_{s,u} instructions. Since these instructions
are no longer prototypes and can now be produced via standard IR, this commit
also removes the target intrinsics and builtins that had been used to prototype
the instructions.

Differential Revision: https://reviews.llvm.org/D100425