runtime: use debugSelect flag to toggle debug code

This block of code once was commented by the original author, but commenting
code looks a little annoying. However, the debugSelect flag is just for the
situation that debug code will be compiled when debuging, when release this
code will be eliminated by the compiler.

Change-Id: I7b94297e368b515116ef44a36058214ddddf9adb
Reviewed-on: https://go-review.googlesource.com/113395Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/select.go

@@ -191,14 +191,15 @@ func selectgo(cas0 *scase, order0 *uint16, ncases int) (int, bool) {
 		}
 		lockorder[j] = o
 	}
-	/*
+
+	if debugSelect {
 		for i := 0; i+1 < ncases; i++ {
 			if scases[lockorder[i]].c.sortkey() > scases[lockorder[i+1]].c.sortkey() {
 				print("i=", i, " x=", lockorder[i], " y=", lockorder[i+1], "\n")
 				throw("select: broken sort")
 			}
 		}
-	*/
+	}
 
 	// lock all the channels involved in the select
 	sellock(scases, lockorder)