[DAG] Create fptoui.sat from clamped fptoui

This is the unsigned variant of D111976, where we convert a clamped
fptoui to a fptoui.sat. Because we are unsigned, the condition this time
is only UMIN of UINT_MAX. Similarly to D111976 it handles ISD::UMIN,
ISD::SETCC/ISD::SELECT, ISD::VSELECT or ISD::SELECT_CC nodes.

This especially helps on ARM/AArch64 where the vcvt instructions
naturally saturate the result.

Differential Revision: https://reviews.llvm.org/D114964