Copy the whole vector and not just VL elements

We were using VADD.VI to do a copy between two physical registers which
should be OK. However we didn't make sure we were using VLMAX to copy
the whole register.

We can't use virtual registers here because Register Allocation has
already run. ExpandPostRA does not behave like PrologEpilogInserter which
runs the scavenger if needed, so we need to explicitly scavenge a couple of
registers.

There is a bit of risk we can't actually scavenge them, in such case we
might want to revisit this and manually spill two registers of our choice
onto the stack.