[ScopInfo] Do not expand getDomains() to full parameter space.

Since several years, isl can perform most operations on sets with differing
parameter spaces, by expanding the parameter space on demand relying using
named isl ids to distinguish different parameter dimensions.

By not always expanding to full dimensionality the set remain smaller and can
likely be operated on faster. This change by itself did not yet result in
measurable performance benefits, but it is a step into the right direction
needed to ensure that subsequent changes indeed can work with lower-dimensional
sets and these sets do not get blown up by accident when later intersected with
the domain context.

llvm-svn: 298053