[RISCV] Update performCombineVMergeAndVOps comments. NFC (#78472)

The current comment was written whenever we had separate TU/TA variants
for
each pseudo, and hasn't been accurate for a while.
This method has grown rather complicated over time so rather than
enumerate all
the different possible cases now (which must be a lot), this updates the
comment to list the different rules that are required for us to be able
to fold
a vmerge.