[TargetLowering] Don't use ISD::SELECT_CC in expandFP_TO_INT_SAT.

This function gets called for vectors and ISD::SELECT_CC was never
intended to support vectors. Some updates were made to support
it when this function started getting used for vectors.

Overall, using separate ISD::SETCC and ISD::SELECT looks like an
improvement even for scalar.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D149481