This reverts commit 79a66ec97b.
The stronger asserts served their purpose; I stumbled across another bug. Will reapply once this one is also fixed.
The bug appears to be a variant of a previous one:
* We mutate an instruction in one block.
* That mutation changes the phase3 results of another block.
This is very similiar to a previous issue, except cross block instead of within a single block.
d4545e6fa0