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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213750 91177308-0d34-0410-b5e6-96231b3b80d8