sim: bfin: stricter insn decoding

We wrote a test case that tries every single 32bit opcode on the hardware
and compared it to the sim.  There were a bunch of places in the sim where
we weren't strict enough (requiring certain parts of the opcode be set) so
we were treating a lot of invalid opcodes as valid ones.  This sprinkles
out a lot additional checks in the dsp32alu class.

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