cmd/compile: run generic deadcode in -N mode

Late opt pass may generate dead stores, which messes up store
chain calculation in later passes. Run generic deadcode even
in -N mode to remove them.

Fixes #26163.

Change-Id: I8276101717bb978d5980e6c7998f53fd8d0ae10f
Reviewed-on: https://go-review.googlesource.com/121856
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/compile.go

@@ -373,7 +373,7 @@ var passes = [...]pass{
 	{name: "softfloat", fn: softfloat, required: true},
 	{name: "late opt", fn: opt, required: true}, // TODO: split required rules and optimizing rules
 	{name: "dead auto elim", fn: elimDeadAutosGeneric},
-	{name: "generic deadcode", fn: deadcode},
+	{name: "generic deadcode", fn: deadcode, required: true}, // remove dead stores, which otherwise mess up store chain
 	{name: "check bce", fn: checkbce},
 	{name: "branchelim", fn: branchelim},
 	{name: "fuse", fn: fuse},

test/fixedbugs/issue26163.go

+// compile -N -d=softfloat
+
+// 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.
+
+// Issue 26163: dead store generated in late opt messes
+// up store chain calculation.
+
+package p
+
+var i int
+var A = ([]*int{})[i]
+
+var F func(float64, complex128) int
+var C chan complex128
+var B = F(1, 1+(<-C))