[AArch64][GlobalISel] Add combine for G_EXTRACT_VECTOR_ELT to allow selection of pairwise FADD.

For the <2 x float> case, instead of adding another combine or legalization to
get it into a <4 x float> form, I'm just adding a GISel specific selection
pattern to cover it.

Differential Revision: https://reviews.llvm.org/D90699