Exploit dereferenceable_or_null attribute in LICM pass

Summary:
Allow hoisting of loads from values marked with dereferenceable_or_null
attribute. For values marked with the attribute perform
context-sensitive analysis to determine whether it's known-non-null or
not.

Patch by Artur Pilipenko!

Reviewers: hfinkel, sanjoy, reames

Reviewed By: reames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9253

llvm-svn: 237593