internal/subtle: s390x rollback first

internal/subtle/xor_s390x.s

@@ -13,39 +13,16 @@ TEXT ·xorBytes(SB),NOSPLIT,$0-32
 	MOVD	n+24(FP), R4
 
 	MOVD	$0, R5
-	CMPBLT	R4, $64, less_than64
-
-loop64b:
-	VLM 0(R2)(R5*1), V0, V3
-	VLM 0(R3)(R5*1), V4, V7
-	VX V0, V4, V4
-	VX V1, V5, V5
-	VX V2, V6, V6
-	VX V3, V7, V7
-	VSTM V4, V7, 0(R1)(R5*1)
-	LAY	64(R5), R5
-	SUB	$64, R4
-	CMPBGE	R4, $64, loop64b
-
-less_than64:
-	CMPBEQ	R4, $0, done // quick end
-	CMPBLT	R4, $32, less_than32
-	VLM 0(R2)(R5*1), V0, V1
-	VLM 0(R3)(R5*1), V2, V3
-	VX V0, V2, V2
-	VX V1, V3, V3
-	VSTM V2, V3, 0(R1)(R5*1)
-	LAY	32(R5), R5
-	SUB	$32, R4
-
-less_than32:
 	CMPBLT	R4, $16, less_than16
+
+loop16b:
 	VL 0(R2)(R5*1), V0
 	VL 0(R3)(R5*1), V1
 	VX V0, V1, V2
 	VST V2, 0(R1)(R5*1)
 	LAY	16(R5), R5
 	SUB	$16, R4
+	CMPBGE	R4, $16, loop16b
 
 less_than16:
 	CMPBLT	R4, $8, tail