[ThinLTO] Ensure we always select the same function copy to import

Summary:
Check if the first eligible callee is under the instruction threshold.
Checking this on the first eligible callee ensures that we don't end
up selecting different callees to import when we invoke this routine
with different thresholds due to reaching the callee via paths that
are shallower or hotter (when there are multiple copies, i.e. with
weak or linkonce linkage). We don't want to leave the decision of which
copy to import up to the backend.

Reviewers: mehdi_amini

Subscribers: inglorion, fhahn, llvm-commits

Differential Revision: https://reviews.llvm.org/D35436

llvm-svn: 308078