sim: bfin: keep track of the exact position of parallel insns

sim: bfin: keep track of the exact position of parallel insns

Some insns need to know which slot they're in to determine whether they
are valid.  So add an enum for each slot, and check that rather than the
overall insn len.  This makes tracking things in the code much clearer.
However, this code is functionally the same, so a follow up patch will
leverage this more to properly flag invalid parallel insn combos.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>