NFC: add early exit in ModuleSummaryAnalysis

Summary:
Changed this code because it was not very readable.
The one question that I got after changing it is, should we
count calls to intrinsics? We don't add them to caller summary,
so maybe we shouldn't also count them?

Reviewers: tejohnson, eraman, mehdi_amini

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23949

llvm-svn: 280036