[dev.ssa] cmd/compile/internal/ssa: fix fallthrough return

Fallthrough return needs to be a return block before jumping
to the exit block.

Change-Id: I994de2064da5c326c9cade2c33cbb15bdbce5acb
Reviewed-on: https://go-review.googlesource.com/14256Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

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

@@ -114,6 +114,7 @@ func buildssa(fn *Node) (ssafn *ssa.Func, usessa bool) {
 
 	// fallthrough to exit
 	if b := s.endBlock(); b != nil {
+		b.Kind = ssa.BlockRet
 		b.AddEdgeTo(s.exit)
 	}