[RISCV] Keep same SEW/LMUL ratio if possible in forward transfer (#69788)

For instructions like vmv.s.x and friends where we don't care about LMUL
or the
SEW/LMUL ratio, we can change the LMUL in its state so that it has the
same
SEW/LMUL ratio as the previous state. This allows us to avoid more VL
toggles
later down the line (i.e. use vsetvli zero, zero, which requires that
the
SEW/LMUL ratio must be the same)

This is an alternative approach to the idea in #69259, but note that
they
don't catch exactly the same test cases.