[LICM] Hoist an invariant_start out of loops if there are no stores executed before it

Once the invariant_start is reached, we know that no instruction *after* it can modify the memory. So, if we can prove the location isn't read *between entry into the loop and the execution of the invariant_start*, we can execute the invariant_start before entering the loop.

Differential Revision: https://reviews.llvm.org/D51181

llvm-svn: 340617