cmd/compile: crash when reporting invalid type alias declaration

[griesemer]

Compiling

package p

func _() {
	type T = T
}

using the old typechecker (-G=0) crashes when reporting an error:

$ go tool compile -G=0 /Users/gri/tmp/x.go
/Users/gri/tmp/x.go:4:7: invalid recursive type alias %!v(PANIC=Format method: runtime error: invalid memory address or nil pointer dereference)
        /Users/gri/tmp/x.go:4:7: %!v(PANIC=Format method: runtime error: invalid memory address or nil pointer dereference) uses %!v(PANIC=Format method: runtime error: invalid memory address or nil pointer dereference)

[gopherbot]

Change https://golang.org/cl/348740 mentions this issue: cmd/compile: don't crash while reporting invalid alias cycle

[gopherbot]

Change https://golang.org/cl/348743 mentions this issue: cmd/compile: fix test case for unified IR (fix build)