local-alloc update_equiv_regs and moving register initialization

[John Wehle]

> a is set and used exactly once, and the set is known to be executed.
> It just happens to come after the use.

Exactly.

> Note that you also have to watch out for cases where the set is statically
> before the use, but dynamically after the use in loop order.

Yes.  I was talking from a CFG point of view.

I'll add moving register initialization within loops to my list of
candidates of things to "play with".  Thanks for pointing out some
of the sharp objects. :-)

-- John
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