cmd/compile: generate more error message for broken type

[cuonglm]

In https://go-review.googlesource.com/c/go/+/191540, @mdempsky suggest to remove this check:

e87fe0f#diff-aaf437751c6b1cd31172b7003e905499R507

With that change, gc will generate more error for test/fixedbugs/issue5172.go:

package main

type foo struct {
	x bar // ERROR "undefined"
}

func main() {
	var f foo
	go f.bar()    // GCCGO_ERROR "undefined"
	defer f.bar() // GCCGO_ERROR "undefined"
}

OTOH, the same behavior with gccgo. #5172 is long time ago and I don't get full context about it.

cc @dominikh

[gopherbot]

Change https://golang.org/cl/191944 mentions this issue: cmd/compile: emit error message for broken type