Refactor graphite-isl-ast-to-gimple.c

Refactor graphite-isl-ast-to-gimple.c:
Refactor so that each function can access 'region'. This will help
maintain a parameter rename_map within a region. No functional change intended.
This patch will be followed by another set of patches
where translate_isl_ast_to_gimple::region is used to keep parameters which need
renaming. Since we are planning to remove limit_scops, we now have to maintain a
set of parameters which needs renaming. This refactoring helps avoid passing
`region' to all the functions in this file.

It passes bootstrap and regtest.

gcc/ChangeLog:

2015-07-19  Aditya Kumar  <hiraditya@msn.com>

        * graphite-isl-ast-to-gimple.c:
	Refactor so that each function can access 'region'. This will help
	maintain a parameter rename_map within a region.

From-SVN: r226014