ARM: try harder to detect non-IT eligible instructions

For many Thumb-1 register register instructions, setting the CPSR is not
permitted inside an IT block.  We would not correctly flag those instructions.
The previous change to identify this scenario was insufficient as it did not
actually catch all the instances.  The current list is formed by manual
inspection of the ARMv6M ARM.

The change to the Thumb2 IT block test is due to the fact that the new more
stringent checking of the MIs results in the If Conversion pass being prevented
from executing (since not all the instructions in the BB are predicable).  This
results in code gen changes.

Thanks to Tim Northover for pointing out that the previous patch was
insufficient and hinting that the use of the v6M ARM would be much easier to use
than the v7 or v8!

llvm-svn: 215382