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

The element index needs to be placed in From3. Before this CL it
was impossible to write a VSTE instruction that could be
successfully parsed, so this won't affect existing assembly code.

Fixes #18075.

Change-Id: I5b71be4c6632b1d5a30820a529122f96fd1bc864
Reviewed-on: https://go-review.googlesource.com/33584
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: Ian Lance Taylor <iant@golang.org>

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

@@ -110,6 +110,8 @@ func IsS390xWithIndex(op obj.As) bool {
 		return true
 	case s390x.AVLEG, s390x.AVLEF, s390x.AVLEH, s390x.AVLEB:
 		return true
+	case s390x.AVSTEG, s390x.AVSTEF, s390x.AVSTEH, s390x.AVSTEB:
+		return true
 	case s390x.AVPDI:
 		return true
 	}

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

@@ -333,7 +333,10 @@ TEXT main·foo(SB),7,$16-0 // TEXT main.foo(SB), 7, $16-0
 	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
-
+	VSTEG	$1, V30, (R1)(R2*1)     // VSTEG   V30, $1, (R1)(R2*1)     // e7e21000180a
+	VSTEF	$3, V2, (R9)            // VSTEF   V2, $3, (R9)            // e7209000300b
+	VSTEH	$7, V31, (R2)           // VSTEH   V31, $7, (R2)           // e7f020007809
+	VSTEB	$15, V29, 4094(R12)     // VSTEB   V29, $15, 4094(R12)     // e7d0cffef808
 
 	RET