[Inliner][NFC] Make test less brittle.
This tests inlining size thresholds, but relies on the output of running
the full O2 pipeline, making it brittle against changes in unrelated
Only run the inlining pass and set thresholds on the test RUN line
Found while investigating D60318.
Reviewers: RKSimon, qcolombet
Differential Revision: https://reviews.llvm.org/D67349