cmd/compile: include constant bools in memcombine

Constant bools are like constant 1-byte values, they memcombine just fine.

(There are still trickier cases that this pass doesn't catch
yet, see TODO at memcombine.go:503.)

Fixes #66413

Change-Id: Ia67cf72ed1c416e27ac22da443bd88a3f09a6cc8
Reviewed-on: https://go-review.googlesource.com/c/go/+/573416
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>

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

@@ -534,7 +534,7 @@ func combineStores(root *Value, n int64) bool {
 	isConst := true
 	for i := int64(0); i < n; i++ {
 		switch a[i].store.Args[1].Op {
-		case OpConst32, OpConst16, OpConst8:
+		case OpConst32, OpConst16, OpConst8, OpConstBool:
 		default:
 			isConst = false
 			break

test/codegen/memcombine.go

@@ -918,3 +918,23 @@ func store16be(p *struct{ a, b uint16 }, x uint32) {
 	// s390x:-"MOVH",-"SRW"
 	p.b = uint16(x)
 }
+
+func storeBoolConst(p *struct{ a, b bool }) {
+	// amd64:"MOVW",-"MOVB"
+	// arm64:"MOVH",-"MOVB"
+	p.a = true
+	p.b = true
+}
+func issue66413(p *struct {
+	a byte
+	b bool
+	c bool
+	d int8
+}) {
+	// amd64:"MOVL",-"MOVB"
+	// arm64:"MOVW",-"MOVB"
+	p.a = 31
+	p.b = false
+	p.c = true
+	p.d = 12
+}