[RISCV] Relax vslide*_vl patterns to allow any mask. NFC (#68203)

We were previously only matching on the true_mask pattern. This patch
allows arbitrary masks to be matched, which means we can avoid the
workaround used in #66267. We can just add patterns for the _MASK pseudo
variants because RISCVDAGToDAGISel::doPeepholeMaskedRVV will transform
them to the unmasked variant if the mask is all ones.