internal/subtle: s390x, check if VLM/VSTM issue

internal/subtle/xor_s390x.s

@@ -10,19 +10,53 @@ TEXT ·xorBytes(SB),NOSPLIT,$0-32
 	MOVD	n+24(FP), R4
 
 	MOVD	$0, R5
-	CMPBLT	R4, $16, tail
+	CMPBLT	R4, $64, tail
 
-loop16b:
+loop_64:
 	VL 0(R2)(R5*1), V0
-	VL 0(R3)(R5*1), V1
-	VX V0, V1, V2
+	VL 16(R2)(R5*1), V1
+	VL 32(R2)(R5*1), V2
+	VL 48(R2)(R5*1), V3
+	VL 0(R3)(R5*1), V4
+	VL 16(R3)(R5*1), V5
+	VL 32(R3)(R5*1), V6
+	VL 48(R3)(R5*1), V7
+	VX V0, V4, V4
+	VX V1, V5, V5
+	VX V2, V6, V6
+	VX V3, V7, V7
+	VST V4, 0(R1)(R5*1)
+	VST V5, 16(R1)(R5*1)
+	VST V6, 32(R1)(R5*1)
+	VST V7, 48(R1)(R5*1)
+	LAY	64(R5), R5
+	SUB	$64, R4
+	CMPBGE	R4, $64, loop_64
+
+tail:
+	CMPBEQ	R4, $0, done
+	CMPBLT	R4, $32, less_than32
+	VL 0(R2)(R5*1), V0
+	VL 16(R2)(R5*1), V1
+	VL 0(R3)(R5*1), V4
+	VL 16(R3)(R5*1), V5
+	VX V0, V2, V2
+	VX V1, V3, V3
 	VST V2, 0(R1)(R5*1)
+	VST V3, 16(R1)(R5*1)
+	LAY	32(R5), R5
+	SUB	$32, R4
+
+less_than32:
+	CMPBLT	R4, $16, less_than16
+	VL 0(R2)(R5*1), V0
+	VL 0(R3)(R5*1), V1
+	VX V0, V1, V1
+	VST V1, 0(R1)(R5*1)
 	LAY	16(R5), R5
 	SUB	$16, R4
-	CMPBGE	R4, $16, loop16b
 
-tail:
-	CMPBEQ	R4, $0, done
+less_than16:	
 	CMPBLT	R4, $8, less_than8
 	MOVD	0(R2)(R5*1), R7
 	MOVD	0(R3)(R5*1), R8