more elaborate jump test

Make the jump test more elaborate with multiple jumps instead
of just one.
add stage selective flush control

Redesign the cpu.v so the stages to be flushed, such as during a branch,
can be selectively chosen.  Currently all of stage 1, 2 and 3 are
flushed for a branch.  But later operations, such as jump, might flush
only stage 1 and 2.