[RISCV] Add a test of vector sadd.overflow to demonstrate intrinsics with...

[RISCV] Add a test of vector sadd.overflow to demonstrate intrinsics with multiple scalable vector results.

This builds on D94142 where scalable vectors are allowed in structs.

I did have to fix one scalable vector issue in the vector type
creation for these intrinsics where we used getVectorNumElements
instead of ElementCount.

Differential Revision: https://reviews.llvm.org/D94149