delocate: eliminate expression from vpaes assembly.

This change eliminates an Aarch64 assembly pattern that only appears
in this case. It's easier to change this code than to pile more things
on top of the delocate parser.

Change-Id: I6bbbe9df744ec2ad4178d74456d8f4fecc3a2dae
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/44845
Reviewed-by: David Benjamin <davidben@google.com>

crypto/fipsmodule/aes/asm/vpaes-armv8.pl

@@ -1059,7 +1059,7 @@ _vpaes_schedule_mangle:
 
 .Lschedule_mangle_both:
 	tbl	v3.16b, {v3.16b}, v1.16b	// vpshufb	%xmm1,	%xmm3,	%xmm3
-	add	x8, x8, #64-16			// add	\$-16,	%r8
+	add	x8, x8, #48			// add	\$-16,	%r8
 	and	x8, x8, #~(1<<6)		// and	\$0x30,	%r8
 	st1	{v3.2d}, [$out]			// vmovdqu	%xmm3,	(%rdx)
 	ret