Fixes the repro added in 947832009997bf9149e88e583c46cc39f6a6136c - previously
when computing mutable ranges of phis, we didn't check that all operands had
been visited. This meant that a backedge could allow a phi's mutable range to
start at 0. Then in PropagateScopeDeps, we might see reject dependencies of a
scope since they appeared to start after a scope — only because the scope's
start was incorrectly too early.
The fix here is to initially set phi.id mutable ranges based on only on operands
that are already visited. Then, during/after the fixpoint iteration of
InferMutableRanges, we start account for all operands since we know they've been
visited at least once and have a real range.