[RISCV] Make sure we use LMUL=1 for scalar reduction input in combineBinOpToReduce

We might have looked through an INSERT_SUBVECTOR to find the
vmv.s.x or vfmv.s.f. If we did the ScalarV type is no longer LMUL=1.
We need to add a new INSERT_SUBVECTOR to restore it before
creating the new reduction.

While there, use the same debug location for all of the newly created
nodes. I believe we were using multiple debug locations from the
original nodes, but changing their relative order. I don't think
we're supposed to do that.