ARM: allow predicated barriers in Thumb mode

The barrier instructions are only "always-execute" in ARM mode, they can quite
happily sit inside an IT block in Thumb.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184964 91177308-0d34-0410-b5e6-96231b3b80d8
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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184965 91177308-0d34-0410-b5e6-96231b3b80d8