cmd/asm: LOOP is a branch instruction on x86

Just a missed case in in the handling of branches.

Fixes #10065

Change-Id: I6be054d30bf1f383c12b4c7626abd5f8ae22b22e
Reviewed-on: https://go-review.googlesource.com/6631
Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/asm/internal/arch/arch.go

@@ -11,6 +11,7 @@ import (
 	"cmd/internal/obj/ppc64"
 	"cmd/internal/obj/x86" // == amd64
 	"fmt"
+	"strings"
 )
 
 // Pseudo-registers whose names are the constant name without the leading R.
@@ -77,7 +78,7 @@ func Set(GOARCH string) *Arch {
 }
 
 func jump386(word string) bool {
-	return word[0] == 'J' || word == "CALL"
+	return word[0] == 'J' || word == "CALL" || strings.HasPrefix(word, "LOOP")
 }
 
 func arch386() *Arch {

src/cmd/asm/internal/asm/testdata/386.out

@@ -45,4 +45,5 @@
 75 00045 (testdata/386.s:75)	CMPPD	X0,foo+4(SB)
 78 00046 (testdata/386.s:78)	PINSRD	(AX),$1,X0
 79 00047 (testdata/386.s:79)	PINSRD	foo+4(FP),$2,X0
-82 00048 (testdata/386.s:82)	RET	,
+83 00048 (testdata/386.s:83)	LOOP	,
+86 00049 (testdata/386.s:86)	RET	,

src/cmd/asm/internal/asm/testdata/386.s

@@ -78,5 +78,9 @@ label:
 	PINSRD	$1, (AX), X0
 	PINSRD	$2, foo+4(FP), X0
 
+// Was bug: LOOP is a branch instruction.
+loop:
+	LOOP	loop
+
 // LTYPE0 nonnon	{ outcode(int($1), &$2); }
 	RET

src/cmd/asm/internal/asm/testdata/amd64.out

@@ -53,4 +53,5 @@
 86 00053 (testdata/amd64.s:86)	PINSRW	R11,$4,AX
 87 00054 (testdata/amd64.s:87)	PINSRW	foo+4(SB),$4,AX
 90 00055 (testdata/amd64.s:90)	RETFL	$4,
-93 00056 (testdata/amd64.s:93)	RET	,
+94 00056 (testdata/amd64.s:94)	LOOP	,
+97 00057 (testdata/amd64.s:97)	RET	,

src/cmd/asm/internal/asm/testdata/amd64.s

@@ -89,5 +89,9 @@ label:
 // LTYPERT spec10	{ outcode($1, &$2); }
 	RETFL	$4
 
+// Was bug: LOOP is a branch instruction.
+loop:
+	LOOP	loop
+
 // LTYPE0 nonnon	{ outcode($1, &$2); }
 	RET