fix(asm): refuse the riscv64 width moves across register banks

Assisted-by: GLM 5.3

asm/riscv_assemble.go

@@ -2095,17 +2095,29 @@ func encodeRISCVMov(instr *ast.Instr, fi riscvFrameInfo, relocs *[]Reloc, lits *
 			// ADDIW $0, rs, rd; the two-operand-only form never has
 			// rd == rs1 in real sources, and the toolchain's C.ADDIW
 			// forbids a zero immediate, so the word stays uncompressed.
+			if srcF || dstF {
+				return nil, fmt.Errorf("%s: expected integer register in rd position", mnem)
+			}
 			return wordLE(riscvIType(riscvEnc{0x1B, 0x0, 0x00}, rd, rs1, 0)), nil
 		case "MOVBU":
 			// ANDI $255, rs, rd; 255 never fits C.ANDI's six signed bits.
+			if srcF || dstF {
+				return nil, fmt.Errorf("%s: expected integer register in rd position", mnem)
+			}
 			return wordLE(riscvIType(riscvEnc{0x13, 0x7, 0x00}, rd, rs1, 0xFF)), nil
 		case "MOVB", "MOVH":
+			if srcF || dstF {
+				return nil, fmt.Errorf("%s: expected integer register in rd position", mnem)
+			}
 			shamt := 56
 			if mnem == "MOVH" {
 				shamt = 48
 			}
 			return riscvExtendBytes(rd, rs1, shamt, true), nil
 		case "MOVHU", "MOVWU":
+			if srcF || dstF {
+				return nil, fmt.Errorf("%s: expected integer register in rd position", mnem)
+			}
 			shamt := 48
 			if mnem == "MOVWU" {
 				shamt = 32