[SCEV] Tweak the output format and content of -analyze

In the "LoopDispositions:" section:

 - Instead of printing out a list, print out a "dictionary" to make it
   obvious by inspection which disposition is for which loop.  This is
   just a cosmetic change.

 - Print dispositions for parent _and_ sibling loops.  I will use this
   to write a test case.

llvm-svn: 268405