RegionGenerator: Do not modify GlobalMaps

By inspection the update of the GlobalMaps in the RegionGenerator seems unneed,
and is removed as also no test cases fail when dropping this. Johannes Doerfert
confirmed that this is indeed save:

"I think that code was needed when we did not use the scalar codegen by default.
Now everything defined in a non-affine region should be communicated via memory
and reloaded in the user block. Hence, we should be good removing this code."

llvm-svn: 246926