fix(arch): keep Zdn out of the class bits of the predicated Z-alias source

Assisted-by: GLM 5.3 Flash

asm/arm64_ext_asm_test.go

@@ -334,6 +334,10 @@ func TestArm64AssembleExtensionStage2Golden(t *testing.T) {
 	}{
 		{"ZABS Z7.D, P4.M, Z13.D", 0x04d6b0ed},
 		{"ZABS Z7.D, P4.Z, Z13.D", 0x04c6b0ed},
+		// The destination read back encodes once, in the destination slot
+		// alone: a non-zero Zdn must stay clear of the class bits.
+		{"ZADD Z15.B, Z5.B, P3.M, Z5.B", 0x04000de5},
+		{"ZSUB Z20.D, Z13.D, P6.M, Z13.D", 0x04c11a8d},
 		{"ZADD Z15.B, Z0.B, P3.M, Z0.B", 0x04000de0},
 		{"ZADD Z7.D, Z23.D, Z13.D", 0x04e702ed},
 		{"ZAND Z15.B, Z0.B, P3.M, Z0.B", 0x041a0de0},