Identify and hoist definitively invariant loads

  As a first step in the direction of assumed invariant loads (loads
  that are not written in some context) we now detect and hoist
  definitively invariant loads. These invariant loads will be preloaded
  in the code generation and used in the optimized version of the SCoP.
  If the load is only conditionally executed the preloaded version will
  also only be executed under the same condition, hence we will never
  access memory that wouldn't have been accessed otherwise. This is also
  the most distinguishing feature to licm.

  As hoisting can make statements empty we will simplify the SCoP and
  remove empty statements that would otherwise cause artifacts in the
  code generation.

Differential Revision: http://reviews.llvm.org/D13194

llvm-svn: 248861