ARM: fix more cases where predication may or may not be allowed

ARM: fix more cases where predication may or may not be allowed

Unfortunately this addresses two issues (by the time I'd disentangled the logic
it wasn't worth putting it back to half-broken):

+ Coprocessor instructions should all be predicable in Thumb mode.
+ BKPT should never be predicable.

llvm-svn: 184965