ConstProp early termination optimization

[johnzl-777]

A (looser) form of this optimization exists in QuEraComputing/bloqade-circuit#736. The idea is to stop constprop from iterating through a loop if loop_vars has converged, preventing a bunch of unnecessary future iterations.

Claude pointed out to me that if there's some dependency on the iteration variable triggering an impure operation in the loop body, the original fix without the guard you currently see would improperly denote the loop as entirely pure.

I already check for no iteration variable dependency in the bloqade-circuit stuff I'm working on but I don't see the harm in having it duplicated here.

@Roger-luo agreed this optimization is worth including in Kirin, with kudos to @weinbe58 and @jasonhan3 for pointing this optimization out to me.

Some benchmarks can be seen here, running against a trivial:

@structural_no_opt
def loop_test(x: int) -> int:
	for _i in range(...):
		x = x + 1
	return x

From range(10) to range(30_000):

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☂️ Python Coverage

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Lines	Covered	Coverage	Threshold	Status
11452	10286	90%	0%	🟢

New Files

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Modified Files

File	Coverage	Status
src/kirin/dialects/scf/constprop.py	91%	🟢
TOTAL	91%	🟢

updated for commit: 582bd7f by action🐍

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