[Inliner][NFC] Make test less brittle.

Summary:
This tests inlining size thresholds, but relies on the output of running
the full O2 pipeline, making it brittle against changes in unrelated
passes.

Only run the inlining pass and set thresholds on the test RUN line
instead.

Found while investigating D60318.

Reviewers: RKSimon, qcolombet

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67349

llvm-svn: 371397