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>

src/cmd/compile/internal/ssa/looprotate.go

@@ -23,6 +23,9 @@ package ssa
 //    JLT loop
 func loopRotate(f *Func) {
 	loopnest := f.loopnest()
+	if loopnest.hasIrreducible {
+		return
+	}
 	if len(loopnest.loops) == 0 {
 		return
 	}