Thumb2 assembler parsing and encoding of IT instruction.

Thumb2 assembler parsing and encoding of IT instruction.

This handles only the handling of the IT instruction itself, not the
processing and validation of the instructions in the IT block. That's next,
and will include encoding tests for IT itself.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@138665 91177308-0d34-0410-b5e6-96231b3b80d8