[RISCV] Use a build_vector instead of a chain insert_vector_elts for vXi1 build_vector lowreing.

A build_vector is the canonical representation rather than multiple
insert_vector_elts.

Unfortunately, this regresses quite a few tests now primarily due to not
having a vmv.s.x special case, but I hope we can improve this with future
patches.

Stress testing in our downstream found an infinite loop in DAG combine.
This patch breaks the infinite loop.

The insert_vector_element chain starts with a fixed vector undef.
Fixed vector undef is currently expanded to a build_vector of 0s
which gets lowered to a vmv.v.i. The insert chain overwrites all
elements so SimplifyDemandedVectorElts turns the vmv.v.i back into
undef and the cycle repeats.

We probably should custom lower fixed vector undef to scalable
vector undef. I think that would also fix the infinite loop, but
I didn't test that.

Reviewed By: luke

Differential Revision: https://reviews.llvm.org/D153399