cmd/compile: obscure initialization loop error message due to inlining

[dvyukov]

go version devel +0a4913b2d4 Wed Aug 23 11:09:11 2017 +0200 linux/amd64

https://play.golang.org/p/TW5DVv3zTk

Produces obscure:

tmp/sandbox278346721/main.go:3:5: initialization loop:
	prog.go:3:5 A refers to
	prog.go:3:5 A

If compiled with -l produces understandable error message:

/tmp/test.go:3:5: initialization loop:
	/tmp/test.go:3:5 A refers to
	/tmp/test.go:7:10 B refers to
	/tmp/test.go:11:10 C refers to
	/tmp/test.go:15:10 D refers to
	/tmp/test.go:3:5 A

In real scenario this is spread across files and intermixed with other code.

[odeke-em]

Looks somewhat like an extension of #16503, but here we get the proper message only with -l. /cc @griesemer and @rsc

[odeke-em]

Thank you @dvyukov for filing this bug, it'll be fixed along with #21752

[griesemer]

Moving to 1.13 like #21752.