This reapplies 427fb5cc5a, which was reverted in 08c1a6b3e1 due to bot
failures.
The fix was to remove an incorrect assertion: In IL_emit, during the initial
worklist loop, an EDU can have all of its dependencies removed without becoming
ready (because it may still have implicit dependencies that will be added back
during the subsequent propagateExtraEmitDeps operation). The EDU will be marked
Ready at the end of IL_emit if its Dependencies set is empty at that point.
Prior to that we can only assert that it's either Emitted or Ready (which is
already covered by other assertions).
6ef4990daa