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

af7925b4 added a custom DAG combine for recognizing fp-to-ints of
extract_subvectors that could be lowered to f64x2.convert_low_i32x4_{s,u}
instructions. This commit extends the combines to recognize equivalent
extract_subvectors of fp-to-ints as well.

Differential Revision: https://reviews.llvm.org/D100790