cmd/compile/internal/ir: add missing import to mknode.go

cmd/compile/internal/ir: add missing import to mknode.go

CL 426257 changed ioutil.WriteFile to os.WriteFile but didn't add the
corresponding import for package os.

Change-Id: I7be32138463b44aabc8e36ce353f6dc1376cbe60
Reviewed-on: https://go-review.googlesource.com/c/go/+/433415
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>