cmd/compile: add test for blank field in composite literal

Updates #18089.

Test for that issue; it was inadvertently fixed
by CL 34988. Ensure that we don't regress on the fix.

Change-Id: Icb85fc20dbb0a47f028f088281319b552b16759d
Reviewed-on: https://go-review.googlesource.com/42173
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/fixedbugs/issue18089.go

@@ -0,0 +1,19 @@
+// compile
+
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package foo
+
+type T struct {
+	x int
+	_ int
+}
+
+func main() {
+	_ = T{0, 0}
+
+	x := T{1, 1}
+	_ = x
+}