Update LLVM to include NullCheckElimination pass

Fixes #11751

llvm @ ab85d02e

-Subproject commit 1bba09755d95892bc826c558630e93803b0a4ee6
+Subproject commit ab85d02e84edeea59ac38505a62ec7d0536cc726

src/rustllvm/llvm-auto-clean-trigger

 # If this file is modified, then llvm will be forcibly cleaned and then rebuilt.
 # The actual contents of this file do not matter, but to trigger a change on the
 # build bots then the contents should be changed so git updates the mtime.
-2014-06-20.2
+2014-07-20