[FIX] Create location if a needed value was not yet demoted

  This allows the code generation to continue working even if a needed
  value (that is reloaded anyway) was not yet demoted. Instead of
  failing it will now create the location for future demotion to memory
  and load from that location. The stores will use the same location and
  by construction execute before the load even if the textual order in
  the generated AST is otherwise.

Reviewers: grosser, Meinersbur

Subscribers: #polly

Differential Revision: http://reviews.llvm.org/D12072

llvm-svn: 245203