This patch to the Go frontend uses the correct initialization order
for code like

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

The global c is initialized by the preinit of b, but we 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.

This fixes https://go.dev/issue/53619.

Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.  Committed
to mainline.

Ian