[AArch64] Improve fptosi.sat vector lowering

Similar to D111236, this improves the lowering of vector fptosi.sat and
fptoui.sat, using legal converts and further saturating from there with
min/max. f64 are excluded for the moment due to producing worse code in
places compared to the unrolling.

Differential Revision: https://reviews.llvm.org/D111787