[SelectionDAG] Fix fptoi.sat scalable vector lowering

Vector fptosi_sat and fptoui_sat were being expanded by unrolling the
vector operation. This doesn't work for scalable vector, so this patch
adds a call to TLI.expandFP_TO_INT_SAT if the vector is scalable.

Scalable tests are added for AArch64 and RISCV. Some of the AArch64
fptoi_sat operations should be legal, but that will be handled in
another patch.

Differential Revision: https://reviews.llvm.org/D130028