[Quake] Fold veq_size through cc.if to activate ForwardEmptyVeqSizePattern

[1tnguyen]

Fixes #4631, where Python slice lowering could leave quake.veq_size on a cc.if result whose else branch yielded cc.undef.

The ForwardEmptyVeqSizePattern did not fire because it saw the cc.if result, not the branch-local cc.undef.

• Add a canonicalization pattern that hoists quake.veq_size through the cc.if by appending the computed size as an extra cc.if result. This exposes the else-branch quake.veq_size(cc.undef) to the existing empty-veq fold, producing 0 for the empty slice case.

• Re-enable the previously disabled cudaq.run slice tests.

Note: I reproduced the segfault on an ARM machine. The issue appears target-sensitive: x86 happened to tolerate the undef value, while ARM did not.

[github-actions]

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[schweitzpgi]

Is the input pattern coming from an explicit place in lowering? Since this is such a special case, I wonder if we can't handle it at the source of the issue.

[1tnguyen]

Indeed, I believe it came from this if/else op:

cuda-quantum/python/cudaq/kernel/ast_bridge.py

Lines 4631 to 4635 in 2a80ac4

	elseBlock = Block.create_at_start(ifOp.elseRegion, [])
	with InsertionPoint(elseBlock):
	subv = cc.UndefOp(self.getVeqType())
	cc.ContinueOp([subv.result])
	self.pushValue(ifOp.result)

I also pondered the idea of changing it at the bridge side, but I couldn't think of a clean solution to support dynamical slice and be compatible with ForwardEmptyVeqSizePattern. Do you have an idea?

[schweitzpgi]

Yeah, that's sort of gross. Eek!

What if the Python bridge looks at the argument its about to use to quake.veq_size, seeing if it is a cc.if and does your rewrite (as in this PR) immediately at that point?

That's sort of odd too, though. Another idea would be to put your pattern in a new Python bridge cleanup pass. I like that a bit better as we can add other "cool" patterns there to cleanup squirrelly residue from the front end. :)

[schweitzpgi]

BTW, ForwardEmptyVeqSizePattern was supposed to kick in after arg synth and aggressive const prop, which would be when the if condition ought to be a constant.

Were you seeing a case where that wasn't true?