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

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214499 91177308-0d34-0410-b5e6-96231b3b80d8