[RISCV] Use DenseMap to track V0 definition. NFC (#84465)

Reviving some of the progress on #71764. To recap, we explored removing
the V0 register copies to simplify the pass, but hit a limitation with
the register allocator due to our use of the vmv0 singleton reg class
and early-clobber constraints.

So since we will have to continue to track the definition of V0
ourselves, this patch simplifies it by storing it in a map. It will
allow us to move about copies to V0 in #71764 without having to do extra
bookkeeping.