size of buffer between PIN/Scarab affects IPC

[bencplin]

Describe the bug
Changing the size of the buffer between PIN/Scarab (by changing the knob KnobMaxBufferSize in the pin_exec pintool) changes the resulting IPC (and other stats). I am filing this as a bug because we currently do not understand why this happens.

To Reproduce
Steps to reproduce the behavior:

1. checkout commit c5b1845
2. run and collect stats, both with KnobMaxBufferSize set to 32, and with KnobMaxBufferSize set to 8, on spec 2017 chckpoints. The exact configuration you run with shouldn't matter, except that wrong path should be enabled (as I suspect the issue is related to wrong path ops)
3. observe the difference in stats produced. Not all of my Spec 2017 checkpoints see a difference, but most do. The IPC different is usually quite small (<0.1%), but non-zero

Expected behavior
We currently expect the exact same IPC, given that all other configurations are the same except the buffer size.