[dev.ssa] cmd/compile: leave JMPs in when using -N

Helps keep line numbers around for debugging, particularly
for break and continue statements (which often compile
down to nothing).

Update #14379

Change-Id: I6ea06aa887b0450d9ba4f11e319e5c263f5a98ba
Reviewed-on: https://go-review.googlesource.com/19848Reviewed-by: David Chase <drchase@google.com>

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

@@ -3627,7 +3627,10 @@ func genssa(f *ssa.Func, ptxt *obj.Prog, gcargs, gclocals *Sym) {
 		}
 		// Emit control flow instructions for block
 		var next *ssa.Block
-		if i < len(f.Blocks)-1 {
+		if i < len(f.Blocks)-1 && Debug['N'] == 0 {
+			// If -N, leave next==nil so every block with successors
+			// ends in a JMP.  Helps keep line numbers for otherwise
+			// empty blocks.
 			next = f.Blocks[i+1]
 		}
 		x := Pc

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

@@ -21,6 +21,7 @@ func critical(f *Func) {
 
 			// allocate a new block to place on the edge
 			d := f.NewBlock(BlockPlain)
+			d.Line = c.Line
 
 			// splice it in
 			d.Preds = append(d.Preds, c)