[RISCV] Combine ({s,u}{div,rem} (zext, zext)) -> (zext ({s,u}{div,rem} (zext, zext))) (#86779)

This narrows unsigned and signed div and rem nodes via
combineBinOpOfZExt.

Unlike other binary ops, there are no widening div or rem instructions.
So we will end up with an extra vzext.vf2.

However I'm assuming that div/rem are expensive enough that by reducing
their EMUL we will gain back the cost.

Alive2 proof: https://alive2.llvm.org/ce/z/Et_L6y