Fix: <rdar://problem/6777209> false Dereference of null pointer in loop:...

Fix: <rdar://problem/6777209> false Dereference of null pointer in loop: pointer increment/decrement preserves non-nullness

When the StoreManager doesn't reason well about pointer-arithmetic, propagate
the non-nullness constraint on a pointer value when performing pointer
arithmetic uisng ++/--.

llvm-svn: 69741