ARM thumb assembly parsing for arithmetic flag setting instructions.

ARM thumb assembly parsing for arithmetic flag setting instructions.

Thumb one requires that many arithmetic instruction forms have an 'S'
suffix. For Thumb2, the whether the suffix is required or precluded depends
on whether the instruction is in an IT block. Use target parser predicates
to check for these sorts of context-sensitive constraints.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@137746 91177308-0d34-0410-b5e6-96231b3b80d8