GPGPU: Use schedule whole components for scheduler

This option increases the scalability of the scheduler and allows us to remove
the 'gisting' workaround we introduced in r275565 to handle a more complicated
test case. Another benefit of using this option is also that the generated
code looks a lot more streamlined.

Thanks to Sven Verdoolaege for reminding me of this option.

llvm-svn: 275573