[RISCV] Explicitly bail if something modifies VL/VTYPE in doLocalPostpass

If an instruction between MI and NextMI uses VL or VTYPE we demand the
respective fields so as to not clobber them at their uses. But we don't
consider if something might modify VL or VTYPE, and will happily coalesce
two vsetvlis when we need to preserve them.

This fixes this by skipping to the next vsetvli. Demanding the fields isn't
enough, as we need to preserve the VL and VTYPE values even if no fields
are demanded.

In practice this doesn't happen, presumably due to there not being any
instructions that write to VL or VTYPE without reading them. But I noticed
this whilst working on a separate patch and split it out.