powerpc/32: Optimise __csum_partial()

Improve __csum_partial by interleaving loads and adds.

On a 8xx, it brings neither improvement nor degradation.
On a 83xx, it brings a 25% improvement.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/lib/checksum_32.S

@@ -47,16 +47,25 @@ _GLOBAL(__csum_partial)
 	bdnz	2b
 21:	srwi.	r6,r4,4		/* # blocks of 4 words to do */
 	beq	3f
+	lwz	r0,4(r3)
 	mtctr	r6
-22:	lwz	r0,4(r3)
 	lwz	r6,8(r3)
+	adde	r5,r5,r0
 	lwz	r7,12(r3)
+	adde	r5,r5,r6
 	lwzu	r8,16(r3)
+	adde	r5,r5,r7
+	bdz	23f
+22:	lwz	r0,4(r3)
+	adde	r5,r5,r8
+	lwz	r6,8(r3)
 	adde	r5,r5,r0
+	lwz	r7,12(r3)
 	adde	r5,r5,r6
+	lwzu	r8,16(r3)
 	adde	r5,r5,r7
-	adde	r5,r5,r8
 	bdnz	22b
+23:	adde	r5,r5,r8
 3:	andi.	r0,r4,2
 	beq+	4f
 	lhz	r0,4(r3)