Add AArch32 DCPS{1,2,3} and HLT instructions.

These were pretty straightforward instructions, with some assembly support
required for HLT.

The ARM assembler is keen to split the instruction mnemonic into a
(non-existent) 'H' instruction with the LT condition code. An exception for
HLT is needed.

HLT follows the same rules as BKPT when in IT blocks, so the special BKPT
hadling code has been adapted to handle HLT also.

Regression tests added including diagnostic tests for out of range immediates
and illegal condition codes, as well as negative tests for pre-ARMv8.

llvm-svn: 190053