[RISCV] Add branch+c.mv macrofusion for sifive-p450. (#76169)

sifive-p450 supports a very restricted version of the short forward
branch optimization from the sifive-7-series.

For sifive-p450, a branch over a single c.mv can be macrofused as a
conditional move operation. Due to encoding restrictions on c.mv, we
can't conditionally move from X0. That would require c.li instead.