fix(asm): reject the shifted-register compositions on loong64

Assisted-by: GLM 5.3

asm/loong64_assemble.go

@@ -487,6 +487,21 @@ func encodeLOONG64Instr(instr *ast.Instr, pc int, offsets map[string]int, fi loo
 	}
 
 	// Pseudo-instructions and the branches first.
+	// Before any of them: the toolchain's loong64 operand grammar has no
+	// shifted-register composition (R0<<2, R0>>R1, R0->3, R0@>3 are parse
+	// errors under GOARCH=loong64 go tool asm), and the shift suffix the
+	// shared parser records is arm64's.  Encoding on would silently drop
+	// the composition and emit the bare register, so every operand whose
+	// verbatim suffix is not the element-selector index (V1.B[3] records
+	// the name as V1.B and the suffix "[3]", which is a real loong64
+	// form) is rejected outright.
+	for _, op := range ops {
+		sfx := strings.Join(strings.Fields(op.Addr.Shift), "")
+		if sfx != "" && !strings.HasPrefix(sfx, "[") {
+			return nil, fmt.Errorf("%s: shifted register operand %s%s is not a loong64 form",
+				mnem, operandRegName(op), sfx)
+		}
+	}
 	switch mnem {
 	case "RET":
 		return loong64Return(fi), nil