[SelectionDAG] Use a VTSDNode to store the saturation width for FP_TO_SINT_SAT/FP_TO_UINT_SAT

Previously we used an i32 constant to store the saturation width, but i32 isn't
legal on RISCV64. This wasn't a big deal to fix, but it is extra work for the
type legalizer.

This patch uses a VTSDNode to store the type similar to SEXT_INREG. This makes
it opaque to the type legalizer.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D101262