cmd/compile/internal/ir: typo

cmd/compile/internal/ir: typo

n.TypeCheck() == 3 is never true now, in n.SetTypecheck(x), when x > 2,
it panic.

Change-Id: Ic876680435d4536eb63dcedee20a07ccdf918b91
Reviewed-on: https://go-review.googlesource.com/c/go/+/504435
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>