BlockGenerator: Make GlobalMap a member variable

The GlobalMap variable used in BlockGenerator should always reference the same
list througout the entire code generation, hence we can make it a member
variable to avoid passing it around through every function call.

History: Before we switched to the SCEV based code generation the GlobalMap
also contained a mapping form old to new induction variables, hence it was
different for each ScopStmt, which is why we passed it as function argument
to copyStmt. The new SCEV based code generation now uses a separate mapping
called LTS -> LoopToSCEV that maps each original loop to a new loop iteration
variable provided as a SCEVExpr. The GlobalMap is currently mostly used for
OpenMP code generation, where references to parameters in the original function
need to be rewritten to the locations of these variables after they have been
passed to the subfunction.

Suggested-by: Johannes Doerfert <doerfert@cs.uni-saarland.de>
llvm-svn: 246920