[BasicAA] Reduce no of map seaches [NFCI].
[BasicAA] Reduce no of map seaches [NFCI].

Summary:
This is a refactoring patch.
- Reduce the number of map searches by reusing the iterator.
- Add asserts to check that the entry is in the cache, as this is something BasicAA relies on to avoid infinite recursion.

Reviewers: chandlerc, aschwaighofer

Subscribers: sanjoy, jlebar, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59151

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@356644 91177308-0d34-0410-b5e6-96231b3b80d8

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