cmd/6c: Improve peep hole optimization of rotate and shift instructions.

Update #4629.

$ cat shift2.c
unsigned int
shift(unsigned int x, unsigned int y)
{
        x = (x << 3);
        y = (y << 5);
        x = (x << 7);
        y = (y << 9);
        return x ^ y;
}

## BEFORE
$ go tool 6c -S shift2.c
(shift2.c:2)	TEXT	shift+0(SB),$0-8
(shift2.c:4)	MOVL	x+0(FP),!!AX
(shift2.c:4)	SALL	$3,!!AX
(shift2.c:4)	MOVL	AX,!!DX
(shift2.c:5)	MOVL	y+4(FP),!!AX
(shift2.c:5)	SALL	$5,!!AX
(shift2.c:5)	MOVL	AX,!!CX
(shift2.c:6)	MOVL	DX,!!AX
(shift2.c:6)	SALL	$7,!!AX
(shift2.c:6)	MOVL	AX,!!DX
(shift2.c:7)	MOVL	CX,!!AX
(shift2.c:7)	SALL	$9,!!AX
(shift2.c:7)	MOVL	AX,!!CX
(shift2.c:8)	MOVL	DX,!!AX
(shift2.c:8)	XORL	CX,!!AX
(shift2.c:8)	RET	,!!
(shift2.c:8)	RET	,!!
(shift2.c:8)	END	,!!

## AFTER
$ go tool 6c -S shift2.c
(shift2.c:2)	TEXT	shift+0(SB),$0-8
(shift2.c:4)	MOVL	x+0(FP),!!AX
(shift2.c:4)	SALL	$3,!!AX
(shift2.c:5)	MOVL	y+4(FP),!!CX
(shift2.c:5)	SALL	$5,!!CX
(shift2.c:6)	SALL	$7,!!AX
(shift2.c:7)	SALL	$9,!!CX
(shift2.c:8)	XORL	CX,!!AX
(shift2.c:8)	RET	,!!
(shift2.c:8)	RET	,!!
(shift2.c:8)	END	,!!

R=rsc, minux.ma, dave, nigeltao
CC=golang-dev
https://golang.org/cl/7066055

src/cmd/6c/peep.c

@@ -330,20 +330,7 @@ subprop(Reg *r0)
 		case AIMULW:
 			if(p->to.type != D_NONE)
 				break;
-
-		case ADIVB:
-		case ADIVL:
-		case ADIVQ:
-		case ADIVW:
-		case AIDIVB:
-		case AIDIVL:
-		case AIDIVQ:
-		case AIDIVW:
-		case AIMULB:
-		case AMULB:
-		case AMULL:
-		case AMULQ:
-		case AMULW:
+			goto giveup;
 
 		case AROLB:
 		case AROLL:
@@ -369,6 +356,23 @@ subprop(Reg *r0)
 		case ASHRL:
 		case ASHRQ:
 		case ASHRW:
+			if(p->from.type == D_CONST)
+				break;
+			goto giveup;
+
+		case ADIVB:
+		case ADIVL:
+		case ADIVQ:
+		case ADIVW:
+		case AIDIVB:
+		case AIDIVL:
+		case AIDIVQ:
+		case AIDIVW:
+		case AIMULB:
+		case AMULB:
+		case AMULL:
+		case AMULQ:
+		case AMULW:
 
 		case AREP:
 		case AREPN:
@@ -384,6 +388,7 @@ subprop(Reg *r0)
 		case AMOVSL:
 		case AMOVSQ:
 		case AMOVQL:
+		giveup:
 			return 0;
 
 		case AMOVL:

src/cmd/6g/peep.c

@@ -664,6 +664,7 @@ subprop(Reg *r0)
 		case AIMULW:
 			if(p->to.type != D_NONE)
 				break;
+			goto giveup;
 
 		case ARCLB:
 		case ARCLL:
@@ -699,6 +700,7 @@ subprop(Reg *r0)
 		case ASHRW:
 			if(p->from.type == D_CONST)
 				break;
+			goto giveup;
 
 		case ADIVB:
 		case ADIVL:
@@ -727,6 +729,7 @@ subprop(Reg *r0)
 		case AMOVSB:
 		case AMOVSL:
 		case AMOVSQ:
+		giveup:
 			if(debug['P'] && debug['v'])
 				print("\tfound %P; return 0\n", p);
 			return 0;