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

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

This recommits 2c51bef7.

I've fixed the broken check line from when I renamed the test function.

Original commit message:
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.