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Propagate values through some wide Zero/Move operations. Among
other things this allows us to optimize some kinds of array
initialization. For example, the following code no longer
requires a temporary be allocated on the stack. Instead it
writes the values directly into the return value.

func f(i uint32) [4]uint32 {
    return [4]uint32{i, i+1, i+2, i+3}
}

The return value is unnecessarily cleared but removing that is
probably a task for dead store analysis (I think it needs to
be able to match multiple Store ops to wide Zero ops).

In order to reliably remove stack variables that are rendered
unnecessary by these new rules I've added a new generic version
of the unread autos elimination pass.

These rules are triggered more than 5000 times when building and
testing the standard library.

Updates #15925 (fixes for arrays of up to 4 elements).
Updates #24386 (fixes for up to 4 kept elements).
Updates #24416.

compilebench results:

name       ol...