vc4: Fix termination of the initial scan for branch targets.

vc4: Fix termination of the initial scan for branch targets.

The loop is scanning until the original max_ip (size of the BO), but we
want to not examine any code after the PROG_END's delay slots.  There was
a block trying to do that, except that we had some early continue
statements if the signal wasn't a PROG_END or a BRANCH.

The failure mode would be that a valid shader is rejected because some
undefined memory after the PROG_END slots is parsed as a branch and the
rest of its setup is illegal.  I haven't seen this in the wild, but
valgrind was complaining and the new userland simulator code started
triggering it.