compiler: Skip copy-backs only if no write to the guard intervenes

[ggorman]

Stacked on #3061; the base switches to main once that merges.

The copy-back skip from #3050 evaluates the later write's guard at the earlier copy-back. If anything that guard reads (indices included) is written in between, the only firing write can be skipped. Single-writer reproducer (every Function has at most one firing write per step, slot and point):

cm = ConditionalDimension(name='cm', parent=time, relation='strict',
                          condition=And(CondEq(time, 3), T[0, 0, 0] < 3.5))
eqs = [Eq(f, T), Eq(T, T + 1), Eq(f, T + 1, implicit_dims=cm)]  # f save=4, T not saved

At time = 3 the increment makes cm false, so only Eq(f, T) fires. f[:, 1, 1] is [0, 1, 2, 3] with opt='noop' and [0, 1, 2, 0] with buffering on main. The indirect-guard variant (the guard reads T through an index Function) gives [2, 3, 4, 5] vs [2, 3, 4, 0].

The skip now applies only if nothing the later guard reads is written between the two copy-backs. Test: test_buffering.py::test_buffering_same_slot_skip[mutable|indirect]; both fail with only buffering.py reverted.

Per the #3050 discussion, the cases with two firing writes to a slot in one step are not used as evidence here.

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