[RISCV][VLOPT] Support vslide{up,down} (#146710)
For vslideup and vslidedown, vl controls the elements which are written
just like other vector instructions. So unless I'm missing something it
should be safe to reduce them. For vslidedown, the specification states
that elements past vl may be read.
We already reduce vslideup and vslidedown in
RISCVVectorPeephole::tryToReduceVL where we just check for
RISCVII::elementsDependOnVL.
Eventually we should replace the whitelist with
RISCVII::elementsDependOnVL once we have test coverage. I've also added
an assert just to double check the instructions we currently support.
This helps reduce vl toggles for fixed-order recurrences vectorized with
EVL tail folding.
