It adds support for negative zero offsets for loads and stores.
Negative zero is returned by the primary expression parser as INT32_MIN, so all that the method needs to do is to accept this value.
Behavior already present for Thumb2.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183734 91177308-0d34-0410-b5e6-96231b3b80d8