cmd/compile: simplify exprformat untyped condition checking

L1337 in fmt.go can be checked just by using "!n.Type.IsUntyped".

Passes toolstash-check.

Change-Id: I5b0c81543bc929367f70713d0ca40b289f905b48
Reviewed-on: https://go-review.googlesource.com/c/go/+/262637


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>