6a/6l: add IMUL3Q and SHLDL

Although Intel considers the three-argument form of IMUL to be a
variant of IMUL, I couldn't make 6l able to differentiate it without
huge changes, so I called it IMUL3.

R=rsc
CC=golang-dev
https://golang.org/cl/5686055

src/cmd/6a/lex.c

@@ -396,6 +396,7 @@ struct
 	"IMULB",	LTYPEI,	AIMULB,
 	"IMULL",	LTYPEI,	AIMULL,
 	"IMULQ",	LTYPEI,	AIMULQ,
+	"IMUL3Q",	LTYPEX,	AIMUL3Q,
 	"IMULW",	LTYPEI,	AIMULW,
 	"INB",		LTYPE0,	AINB,
 	"INL",		LTYPE0,	AINL,

src/cmd/6l/6.out.h

@@ -735,6 +735,7 @@ enum	as
 	AMODE,
 	ACRC32B,
 	ACRC32Q,
+	AIMUL3Q,
 
 	ALAST
 };

src/cmd/6l/optab.c

@@ -267,6 +267,11 @@ uchar	yimul[] =
 	Yml,	Yrl,	Zm_r,	2,
 	0
 };
+uchar	yimul3[] =
+{
+	Yml,	Yrl,	Zibm_r,	1,
+	0
+};
 uchar	ybyte[] =
 {
 	Yi64,	Ynone,	Zbyte,	1,
@@ -772,6 +777,7 @@ Optab optab[] =
 	{ AIMULL,	yimul,	Px, 0xf7,(05),0x6b,0x69,Pm,0xaf },
 	{ AIMULQ,	yimul,	Pw, 0xf7,(05),0x6b,0x69,Pm,0xaf },
 	{ AIMULW,	yimul,	Pe, 0xf7,(05),0x6b,0x69,Pm,0xaf },
+	{ AIMUL3Q,	yimul3,	Pw, 0x6b },
 	{ AINB,		yin,	Pb, 0xe4,0xec },
 	{ AINCB,	yincb,	Pb, 0xfe,(00) },
 	{ AINCL,	yincl,	Px, 0xff,(00) },