[CostModel] Add basic fptoi_sat costs

This adds some basic fptosi_sat and fptoui_sat target independent cost
modelling. The fptosi_sat is modelled as a fmin/fmax to saturate the
value, followed by a fp convert. The signed values then have an
additional fcmp+select for handling Nan correctly.

The AArch64/Arm costs may be more incorrect, as the instruction exist
natively. This can be fixed with target specific cost updates.

Differential Revision: https://reviews.llvm.org/D124269