I'd made this change to my own 6l copy, once I'd noticed it,
but then forgot to pass it on:
http://code.google.com/p/go/source/detail?r=060260a3f8

Reference: /n/sources/patch/applied/6l-movlqzx
Date: Sun Nov 18 00:55:31 CET 2012
Signed-off-by: charles.forsyth@gmail.com

--- /sys/src/cmd/6l/optab.c	Sun Nov 18 00:54:17 2012
+++ /sys/src/cmd/6l/optab.c	Sun Nov 18 00:54:13 2012
@@ -762,7 +762,7 @@
 	{ AMOVLPD,	yxmov,	Pe, 0x12,0x13 },
 	{ AMOVLPS,	yxmov,	Pm, 0x12,0x13 },
 	{ AMOVLQSX,	yml_rl,	Pw, 0x63 },
-	{ AMOVLQZX,	yml_rl,	Px, 0x63 },
+	{ AMOVLQZX,	yml_rl,	Px, 0x8b },
 	{ AMOVMSKPD,	yxrrl,	Pq, 0x50 },
 	{ AMOVMSKPS,	yxrrl,	Pm, 0x50 },
 	{ AMOVNTO,	yxr_ml,	Pe, 0xe7 },