Fixes#137509.
When `op->fold` returns a Value that is not one of `op`'s operands (e.g.
`unrealized_conversion_cast`'s fold returns the inner cast's operand),
`SparseConstantPropagation` read that value's lattice without
subscribing to it -- so the op was not revisited when the lattice
widened and its stale fold result was not updated.
Fix by using `getLatticeElementFor(getProgramPointAfter(op), v)` to
register the dependency. This matches a few places in
`SparseAnalysis.cpp` where the same strategy is used.
I'd love to use something even simpler than `unrealized_conversion_cast`
operation in the test, but this is what i got when minimizing the
reproduction from the original issue (#137509) and i wasn't able to find
any operation that would work for this reproduction.
Assisted-By: Claude Code
6f1e6e47bd