[RISCV] Select ISD::AVGCEILS/AVGFLOORS as vaadd. (#92839)

I think the behaviors are the same if this describes their behavior.

AVGFLOORS sign extends the inputs by 1 bit, adds them, then does an
arithmetic shift right by 1 before truncating to the original bit width.
This is vaadd with rdn rounding mode.

AVGCEILS sign extends the inputs by 1 bit, adds them, then does an
arithmetic shift right by 1. If the bit shifted out is 1, it adds 1 to
the shifted value. Then truncates to the original bit width. This is vaadd
with rnu rounding mode.

I think this wasn't implemented previously because there was some
confusion about what average means. Some may expect average to round
towards zero, but there is no way to do that in RISC-V or with the
SelectionDAG nodes. Related issue
https://github.com/riscv/riscv-v-spec/issues/935