s390x-mont.pl: optimize prologue.

crypto/bn/asm/s390x-mont.pl

@@ -64,7 +64,7 @@ bn_mul_mont:
 	sla	$num,3		# $num to enumerate bytes
 	la	$bp,0($num,$bp)
 
-	stmg	%r2,%r15,16($sp)
+	stg	%r2,16($sp)
 
 	cghi	$num,16		#
 	lghi	%r2,0		#
@@ -72,6 +72,8 @@ bn_mul_mont:
 	cghi	$num,128	#
 	bhr	%r14		# if($num>128) return 0;
 
+	stmg	%r3,%r15,24($sp)
+
 	lghi	$rp,-160-8	# leave room for carry bit
 	lcgr	$j,$num		# -$num
 	lgr	%r0,$sp