cmd/compile: global variable initialization done in unexpected order [1.18 backport]

[gopherbot]

@mdempsky requested issue #51913 to be considered for backport to the next 1.18 minor release.

@gopherbot Please backport to Go 1.18. This is a silent miscompilation of valid (albeit unlikely) code.

[mdempsky]

The issue is that https://go.dev/play/p/vyyJGI51r2w should pass, but it fails with Go 1.18 (and Go 1.17). It's a somewhat obscure case IMO (gccgo added the same broken optimization to match cmd/compile semantics actually), but it is in fact valid code.

https://go.dev/cl/395541 should backport cleanly and fixes the issue. The other package runtime CLs are not necessary for the fix.

[toothrot]

@mdempsky Should we open a backport for Go 1.17 as well?