cmd/compile: finish GOEXPERIMENT=preemptibleloops repair

A newish check for branch-likely on single-successor blocks
caught a case where the preemption-check inserter was
setting "likely" on an unconditional branch.

Fixed by checking for that case before setting likely.

Also removed an overconservative restriction on parallel
compilation for GOEXPERIMENT=preemptibleloops; it works
fine, it is just another control-flow transformation.

Change-Id: I8e786e6281e0631cac8d80cff67bfb6402b4d225
Reviewed-on: https://go-review.googlesource.com/102317
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/compile/internal/gc/main.go

@@ -1223,7 +1223,7 @@ func concurrentBackendAllowed() bool {
 		return false
 	}
 	// TODO: Test and delete these conditions.
-	if objabi.Fieldtrack_enabled != 0 || objabi.Preemptibleloops_enabled != 0 || objabi.Clobberdead_enabled != 0 {
+	if objabi.Fieldtrack_enabled != 0 || objabi.Clobberdead_enabled != 0 {
 		return false
 	}
 	// TODO: fix races and enable the following flags

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

@@ -179,7 +179,9 @@ func insertLoopReschedChecks(f *Func) {
 		if p.i != 0 {
 			likely = BranchUnlikely
 		}
-		bb.Likely = likely
+		if bb.Kind != BlockPlain { // backedges can be unconditional. e.g., if x { something; continue }
+			bb.Likely = likely
+		}
 
 		// rewrite edge to include reschedule check
 		// existing edges: