[LICM] Allow AliasSetMap to contain top-level loops.

When an outer loop gets deleted by a different pass, before LICM visits
it, we cannot clean up its sub-loops in AliasSetMap, because at the
point we receive the deleteAnalysisLoop callback for the outer loop, the loop
object is already invalid and we cannot access its sub-loops any longer.

Reviewers: asbirlea, sanjoy, chandlerc

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D61904

llvm-svn: 360704