Add support for scalar dependences to OpenMP code generation

Scalar dependences between scop statements have caused troubles during parallel
code generation as we did not pass on the new stack allocation created for such
scalars to the parallel subfunctions. This change now detects all scalar
reads/writes in parallel subfunctions, creates the allocas for these scalar
objects, passes the resulting memory locations to the subfunctions and ensures
that within the subfunction requests for these memory locations will return the
rewritten values.

Johannes suggested as a future optimization to privatizing some of the scalars
in the subfunction.

llvm-svn: 246414