This reverts commit 35dacf2f51.
And relands the original change with two additions so I can debug the failure on Arm/AArch64:
* Enable lldb step logging in the tests.
* Assert that the current plan is not the base plan at the spot I believe is calling PopPlan.
These will be removed and replaced with a proper fix once I see some failures on the bots,
I couldn't reproduce it locally.
(also, no sign of it on the x86_64 bot)