cmd/compile: don't initialize blank struct fields

We already skipped blank field initialization in non-global contexts.
This change makes the global context treatment match.

Fixes #31546

Change-Id: I40acce49b0a9deb351ae0da098f4c114e425ec63
Reviewed-on: https://go-review.googlesource.com/c/go/+/173723
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/sinit.go

@@ -1278,6 +1278,9 @@ func (s *InitSchedule) initplan(n *Node) {
 			if a.Op != OSTRUCTKEY {
 				Fatalf("initplan structlit")
 			}
+			if a.Sym.IsBlank() {
+				continue
+			}
 			s.addvalue(p, a.Xoffset, a.Left)
 		}
 

test/fixedbugs/issue31546.go

@@ -0,0 +1,20 @@
+// run
+
+// Copyright 2019 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 main
+
+import (
+	"reflect"
+)
+
+var x = struct{ a, _, c int }{1, 2, 3}
+
+func main() {
+	if i := reflect.ValueOf(x).Field(1).Int(); i != 0 {
+		println("got", i, "want", 0)
+		panic("fail")
+	}
+}