cmd/compile: fix bug in phiopt pass

The math to invert the input index was wrong.

Fixes #45323

Change-Id: I7c68cac280e8f01a9c806ecb0f195f169267437e
Reviewed-on: https://go-review.googlesource.com/c/go/+/306431
Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: fannie zhang <Fannie.Zhang@arm.com>
Reviewed-by: David Chase <drchase@google.com>

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

@@ -213,7 +213,7 @@ func phiopt(f *Func) {
 				ei := b.Preds[1].i
 				sb0 := pb1.Succs[1-ei].b
 				if sdom.IsAncestorEq(sb0, pb0) {
-					convertPhi(pb1, v, ei-1)
+					convertPhi(pb1, v, 1-ei)
 					break
 				}
 			} else {

test/fixedbugs/issue45323.go

@@ -0,0 +1,24 @@
+// compile
+
+// Copyright 2021 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
+
+func g() bool
+
+func f(y int) bool {
+	b, ok := true, false
+	if y > 1 {
+		ok = g()
+	}
+	if !ok {
+		ok = g()
+		b = false
+	}
+	if !ok {
+		return false
+	}
+	return b
+}