ARM: implement support for the UDF mnemonic

The UDF instruction is a reserved undefined instruction space.  The assembler
mnemonic was introduced with ARM ARM rev C.a.  The instruction is not predicated
and the immediate constant is ignored by the CPU.  Add support for the three
encodings for this instruction.

The changes to the invalid instruction test is due to the fact that the invalid
instructions actually overlap with the undefined instruction.  Introduction of
the new instruction results in a partial decode as an undefined sequence.  Drop
the tests as they are invalid instruction patterns anyways.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208751 91177308-0d34-0410-b5e6-96231b3b80d8