Summary:
Don't assign values to undefined references, simply don't emit those
reference edges as they are not useful (we were already not emitting
call edges to undefined refs).
Also, streamline the later lookup of value ids when writing the
summaries, by combining the check for value id existence with the access
of that value id.
Reviewers: pcc
Subscribers: Prazek, llvm-commits, inglorion
Differential Revision: https://reviews.llvm.org/D33634
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@304323 91177308-0d34-0410-b5e6-96231b3b80d8
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