[RISCV] Combine extract_vector_elt followed by VFMV_S_F_VL.

If we're extracting an element and inserting into a undef vector
with the same number of elements, we can use the original vector.

This pattern occurs around reductions that have been cascaded
together.

This can be generalized to wider/narrow vectors by using
insert_subvector/extract_subvector, but we don't have lit tests
for that case currently.

We can also support non-undef before by using a slide or vmv.v.v

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D142264