cmd/asm: fix bug about VMOV instruction (move a vector element to another) on ARM64

This change fixes index error when encoding VMOV instruction which pattern
is vmov Vn.<T>[index], Vd.<T>[index]

Change-Id: I949166e6dfd63fb0a9365f183b6c50d452614f9d
Reviewed-on: https://go-review.googlesource.com/101335Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/asm/internal/asm/testdata/arm64.s

@@ -239,6 +239,7 @@ TEXT	foo(SB), DUPOK|NOSPLIT, $-8
 	VMOV	V10.S[0], V12.S[1]    // 4c050c6e
 	VMOV	V9.H[0], V12.H[1]     // 2c05066e
 	VMOV	V8.B[0], V12.B[1]     // 0c05036e
+	VMOV	V8.B[7], V4.B[8]      // 043d116e
 	VREV32	V5.B16, V5.B16        // a508206e
 	VDUP	V19.S[0], V17.S4      // 7106044e
 //

src/cmd/internal/obj/arm64/asm7.go

@@ -3910,7 +3910,7 @@ func (c *ctxt7) asmout(p *obj.Prog, o *Optab, out []uint32) {
 		default:
 			c.ctxt.Diag("invalid arrangement: %v", p)
 		}
-		o1 |= (uint32(imm5&0x1f) << 16) | (uint32(imm4&0xf) << 16) | (uint32(rf&31) << 5) | uint32(rt&31)
+		o1 |= (uint32(imm5&0x1f) << 16) | (uint32(imm4&0xf) << 11) | (uint32(rf&31) << 5) | uint32(rt&31)
 
 		break