[ThinLTO] Reduce unnecessary map lookups during combined summary write

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

llvm-svn: 304323