rework the ownership of subloop alias information: instead of

rework the ownership of subloop alias information: instead of
keeping them around until the pass is destroyed, keep them
around a) just when useful (not for outer loops) and b) destroy
them right after we use them.  This should reduce memory use
and fixes potential bugs where a loop is deleted and another
loop gets allocated to the same address.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@112446 91177308-0d34-0410-b5e6-96231b3b80d8