[ARM] Cost modelling for scalar fptoi_sat

Similar to D124357, this adds some cost modelling for fptoi_sat for Arm
targets. Where VFP2 is available (and FP64/FP16 for the relevant types),
the operations are legal as the Arm instructions naturally saturate.
Otherwise they will need an extra smin/smax clamp, similar to AArch64.

Differential Revision: https://reviews.llvm.org/D125665