[dev.ssa] cmd/compile: reenable TestStackBarrierProfiling

Tested it 1000x on OS X and Linux amd64, no failures.
Updated TODO.

Change-Id: Ia60c8d90962f6e5f7c3ed1ded6ba1b25eee983e1
Reviewed-on: https://go-review.googlesource.com/19662
Reviewed-by: Todd Neal <todd@tneal.org>

src/cmd/compile/internal/ssa/TODO

@@ -7,7 +7,6 @@ Coverage
 Correctness
 -----------
 - Debugging info (check & fix as much as we can)
-- Re-enable TestStackBarrierProfiling (src/runtime/pprof/pprof_test.go)
 - @ directive in rewrites might read overwritten data.  Save @loc
   in variable before modifying v.
 

src/runtime/pprof/pprof_test.go

@@ -375,11 +375,6 @@ func TestStackBarrierProfiling(t *testing.T) {
 		t.Skipf("low resolution timers inhibit profiling signals (golang.org/issue/13405)")
 		return
 	}
-	if true {
-		// TODO(khr): remove
-		t.Skipf("skipping for SSA branch, flaky")
-		return
-	}
 
 	if !strings.Contains(os.Getenv("GODEBUG"), "gcstackbarrierall=1") {
 		// Re-execute this test with constant GC and stack