cmd/compile: add logging to critical and phielim

Change-Id: Ieefeceea40bd29657fd519368b0920dad8443844
Reviewed-on: https://go-review.googlesource.com/20712
Run-TryBot: Todd Neal <todd@tneal.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -22,6 +22,9 @@ func critical(f *Func) {
 			// allocate a new block to place on the edge
 			d := f.NewBlock(BlockPlain)
 			d.Line = c.Line
+			if f.pass.debug > 0 {
+				f.Config.Warnl(c.Line, "split critical edge")
+			}
 
 			// splice it in
 			d.Preds = append(d.Preds, c)

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

@@ -64,5 +64,9 @@ func phielimValue(v *Value) bool {
 	}
 	v.Op = OpCopy
 	v.SetArgs1(w)
+	f := v.Block.Func
+	if f.pass.debug > 0 {
+		f.Config.Warnl(v.Line, "eliminated phi")
+	}
 	return true
 }