BOLT expects pre-aggregated profile entries to be unique, which holds
for externally aggregated traces (or branches+fall-through ranges).
Therefore, BOLT doesn't merge duplicate entries for faster processing.
However, such traces are not expressly prohibited and could come from
concatenated pre-aggregated profiles or otherwise.
Relax the assumption about no duplicate (branch-only) traces in fall-
through imputing.
Test Plan: updated callcont-fallthru.s
1d0aa6c2ad