cmd/asm: fix element size encoding for VSUMQ instruction on s390x

The element size for VSUMQF and VSUMQG was off by one. Fix this
and add tests for VSUM* instruction encodings.

Change-Id: I6de2dabb383e5bc6f85eef1e0f106ba949c9030b
Reviewed-on: https://go-review.googlesource.com/c/go/+/199978
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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

@@ -435,6 +435,12 @@ TEXT main·foo(SB),DUPOK|NOSPLIT,$16-0 // TEXT main.foo(SB), DUPOK|NOSPLIT, $16-
 	VMSLEG  V21, V22, V23, V24      // e78563807fb8
 	VMSLOG  V21, V22, V23, V24      // e78563407fb8
 	VMSLEOG V21, V22, V23, V24      // e78563c07fb8
+	VSUMGH	V1, V2, V3              // e73120001065
+	VSUMGF	V16, V17, V18           // e72010002e65
+	VSUMQF	V4, V5, V6              // e76450002067
+	VSUMQG	V19, V20, V21           // e75340003e67
+	VSUMB	V7, V8, V9              // e79780000064
+	VSUMH	V22, V23, V24           // e78670001e64
 
 	RET
 	RET	foo(SB)

src/cmd/internal/obj/s390x/vector.go

@@ -978,9 +978,9 @@ func vop(as obj.As) (opcode, es, cs uint32) {
 	case AVSUMQ:
 		return op_VSUMQ, 0, 0
 	case AVSUMQF:
-		return op_VSUMQ, 1, 0
-	case AVSUMQG:
 		return op_VSUMQ, 2, 0
+	case AVSUMQG:
+		return op_VSUMQ, 3, 0
 	case AVSUM:
 		return op_VSUM, 0, 0
 	case AVSUMB: