[RISCV] Add tests showing missed vector saturating add/sub combines

These will be optimized by upcoming patches. The tests are primarily not
being optimized due to the lack of support for saturating vector
arithmetic in the RISC-V backend.

On top of that, however, a large percentage of the scalable-vector tests
are also lacking support in the DAGCombiner: either in
`ISD::matchBinaryPredicate` or due to checks specifically for
`BUILD_VECTOR` and not `SPLAT_VECTOR`.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D106649