cmd/compile: move Strongly Connected Components code into new file

This logic is used by the current escape analysis pass, but otherwise
logically independent. Move (unchanged) into a separate file to make
that clearer, and to make it easier to replace esc.go later.

Updates #23109.

Change-Id: Iec8c0c47ea04c0008165791731c11d9104d5a474
Reviewed-on: https://go-review.googlesource.com/c/go/+/167715


Reviewed-by: Robert Griesemer <gri@golang.org>