[ARM] Make the assembler reject unpredictable pre/post-indexed ARM STRH instructions.

The ARM ARM prohibits STRH instructions with writeback into the source register. With this commit this constraint is now enforced and we stop assembling STRH instructions with unpredictable behavior.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213850 91177308-0d34-0410-b5e6-96231b3b80d8