cmd/compile: make loop finder more aware of irreducible loops

cmd/compile: make loop finder more aware of irreducible loops

The loop finder doesn't return good information if it
encounters an irreducible loop.  Make a start on improving
this, and set a function-level flag to indicate when there
is such a loop (and the returned information might be flaky).

Use that flag to prevent the loop rotater from getting
confused; the existing code seems to depend on artifacts
of the previous loop-finding algorithm. (There is one
irreducible loop in the go library, in "inflate.go").

Change-Id: If6e26feab38d9b009d2252d556e1470c803bde40
Reviewed-on: https://go-review.googlesource.com/42150
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>