CodeGen: Allow function parameters to be rewritten in getNewValue()

When deriving new values for the statements of a SCoP, we assumed that parameter
values are constant within the SCoP and consquently do not need to be rewritten.
For OpenMP code generation this assumption is wrong, as such values are not
available in the OpenMP subfunction and consequently also may need to be
rewritten.

Committed with some changes.

Contributed-By: Johannes Doerfert <s9jodoer@stud.uni-saarland.de>
llvm-svn: 153838