ScopInfo: use project_out instead of remove_dims

By just removing dimensions (and the constraints they are involved in) we
may loose information about the dimensions we do not remove. By instead
using project_out, we are sure all constraints on the outer dimensions are
preserved.

No test case, as this error condition is very unlikely to be triggered by
isl's current code. We still 'fix' this, as isl gives little guarantees
regarding the behavior of remove_divs.

llvm-svn: 246567