compiler: use correct init order for multi-value initialization

compiler: use correct init order for multi-value initialization

Use the correct initialization order for

var a = c
var b, c = x.(bool)

The global c is initialized by the preinit of b, but were missing a
dependency of c on b, so a would be initialized to the zero value of c
rather than the correct value.

Simply adding the dependency of c on b didn't work because the preinit
of b refers to c, so that appeared circular.  So this patch changes
the init order to skip dependencies that only appear on the left hand
side of assignments in preinit blocks.

Doing that didn't work because the write barrier pass can transform "a
= b" into code like "gcWriteBarrier(&a, b)" that is not obviously a
simple assigment.  So this patch moves the collection of dependencies
to just after lowering, before the write barriers are inserted.

Making those changes permit relaxing the requirement that we don't
warn about self-dependency in preinit blocks, so now we correctly warn
for

var a, b any = b.(bool)

The test case is https://go.dev/cl/415238.

Fixes golang/go#53619

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/415594