[ScopInfo] Do not use the set dimension ids to carry loop information

isl does not guarantee that set dimension ids will be preserved, so using them
to carry information is not a good idea. Furthermore, the loop information can
be derived without problem from the statement itself. As this even requires
less code than propagating loop information on set dimension ids, starting from
this commit we just derive the loop information in collectSurroundingLoops
directly from the IR.

Interestingly this also results in a couple of isl sets to take a simpler
representation.

llvm-svn: 326664