cmd/asm: fix parsing of the s390x VLE{G,F,H,B} instructions

This commit makes the assembler frontend reorder the operands so that
they are in the order the backend expects. The index should be first
for consistency with the other vector instructions.

Before this commit no operand order would have been accepted so this
isn't a breaking change.

Change-Id: I188d57eeb338d27fa1fa6845de0d6d1521b7a6c3
Reviewed-on: https://go-review.googlesource.com/29855
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bill O'Farrell <billotosyr@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/asm/internal/arch/s390x.go

@@ -112,6 +112,8 @@ func IsS390xWithIndex(op obj.As) bool {
 		return true
 	case s390x.AVLEIG, s390x.AVLEIF, s390x.AVLEIH, s390x.AVLEIB:
 		return true
+	case s390x.AVLEG, s390x.AVLEF, s390x.AVLEH, s390x.AVLEB:
+		return true
 	case s390x.AVPDI:
 		return true
 	}

src/cmd/asm/internal/asm/testdata/s390x.s

@@ -257,6 +257,11 @@ TEXT main·foo(SB),7,$16-0 // TEXT main.foo(SB), 7, $16-0
 	WFMSDB	V2, V25, V24, V31       // WFMSDB  V25, V24, V2, V31       // e7f298038b8e
 	VPERM	V31, V0, V2, V3         // VPERM   V0, V2, V31, V3         // e73f0000248c
 	VPDI	$1, V2, V31, V1         // VPDI    V2, V31, $1, V1         // e712f0001284
+	VLEG	$1, (R3), V1            // VLEG    (R3), $1, V1            // e71030001002
+	VLEF	$2, (R0), V31           // VLEF    (R0), $2, V31           // e7f000002803
+	VLEH	$3, (R12), V16          // VLEH    (R12), $3, V16          // e700c0003801
+	VLEB	$15, 4095(R9), V15      // VLEB    4095(R9), $15, V15      // e7f09ffff000
+
 
 	RET