execute_alu: bypass a/b operand muxes for div operations

Divide operations on immediates are not supported,
so a small optimisation is to use the register inputs
directly instead of goiing through the operand muxes.
execute_alu: fix typo
cappuccino/ctrl: fix stepped_into_delay_slot bug

the delay_slot signal from control stage should be used,
not the one in execute stage
cappuccino/ctrl: move branch insn decoding to mor1kx_decode