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 <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go