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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213745 91177308-0d34-0410-b5e6-96231b3b80d8