[RISCV][VLOPT] Support v[f]slide1up.v{x,f} (#146716)
Similarly to #146710, for vslide1ups vl only determines the destination
elements written to so we can safely reduce their AVL.
We cannot do this for vslide1downs as the vl determines which lane the
new element is to be inserted in, so some negative tests have been
added.
