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

Assisted-by: GLM 5.3 Flash

arch/arm64_ext.go

@@ -573,8 +573,9 @@ const (
 	// ExtFormZPredicatedSource is the Z-alias binary predicated form,
 	// ZADD Z15.B, Z0.B, P3.M, Z0.B: the destination's source copy is
 	// spelled and must equal the destination.  Operands: Zm.T, Zdn.T,
-	// Pg/M, Zdn.T.  Fields: Zm in the second-source slot, Zdn in the
-	// first-source slot, Pg narrow, Zdn in the destination.
+	// Pg/M, Zdn.T.  Fields: Zm in the first-source slot at bits 9..5, Pg
+	// narrow, Zdn in the destination slot; bits 16..20 are class bits no
+	// operand touches.
 	ExtFormZPredicatedSource
 	// ExtFormZImmediateSource is the Z-alias immediate form, ZADD $6,
 	// Z7.D, Z7.D: the destination's source copy is spelled and must equal
@@ -1787,6 +1788,8 @@ func (in ExtInstr) encodeZUnary(ops []ExtOperand) ([]byte, error) {
 
 // encodeZPredicatedSource fills the Z-alias binary predicated form: Zm.T,
 // Zdn.T, Pg/M, Zdn.T, the source copy spelled and equal to the destination.
+// Zm rides the first-source slot at bits 9..5 and Zdn the destination slot;
+// bits 16..20 carry the class opcode and stay clear of operands.
 func (in ExtInstr) encodeZPredicatedSource(ops []ExtOperand) ([]byte, error) {
 	zm, zdn, pg := ops[0], ops[1], ops[2]
 	if err := in.zVector(zm, 1); err != nil {
@@ -1819,7 +1822,6 @@ func (in ExtInstr) encodeZPredicatedSource(ops []ExtOperand) ([]byte, error) {
 	}
 	word := in.Word
 	word = extSet(word, extFieldRn, uint32(zm.Reg))
-	word = extSet(word, extFieldRm, uint32(zdn.Reg))
 	word = extSet(word, extFieldPgN, uint32(pg.Reg))
 	word = extSet(word, extFieldRd, uint32(zdn.Reg))
 	if size != 0 {