cmd/compile: fix inlining of constant if statements

We accidentally overlooked needing to still visit Ninit for OIF
statements with constant conditions in golang.org/cl/96778.

Fixes #24120.

Change-Id: I5b341913065ff90e1163fb872b9e8d47e2a789d2
Reviewed-on: https://go-review.googlesource.com/97475
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -363,7 +363,8 @@ func (v *hairyVisitor) visit(n *Node) bool {
 	case OIF:
 		if Isconst(n.Left, CTBOOL) {
 			// This if and the condition cost nothing.
-			return v.visitList(n.Nbody) || v.visitList(n.Rlist)
+			return v.visitList(n.Ninit) || v.visitList(n.Nbody) ||
+				v.visitList(n.Rlist)
 		}
 	}
 

test/fixedbugs/issue24120.go

+// compile
+
+// Copyright 2018 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 p
+
+var F func(int)
+
+func G() {
+	if F(func() int { return 1 }()); false {
+	}
+}