[RISCV] Add support for vector saturating add/sub operations

This patch adds support for lowering the saturating vector add/sub
intrinsics to RVV instructions, for both fixed-length and
scalable-vector forms alike.

Note that some of the DAG combines are still not triggering for the
scalable-vector tests. These require a bit more work in the DAGCombiner
itself.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D106651