fix(asm): lower the riscv64 immediate CSR pseudos onto their opcode forms

Assisted-by: GLM 5.3

asm/riscv_assemble.go

@@ -1532,6 +1532,21 @@ func encodeRISCVInstr(instr *ast.Instr, pc int, offsets map[string]int, fi riscv
 				csrOp, srcOp = ops[1], ops[0]
 			}
 			rdOp = nil
+			// The toolchain picks the opcode form from the source's kind:
+			// CSRW $2, csr assembles as CSRRWI exactly as CSRWI does, and
+			// the register spellings stay on CSRRW.
+			if isImmOperand(srcOp) {
+				csrMnem = map[string]string{
+					"CSRS": "CSRRSI", "CSRW": "CSRRWI", "CSRC": "CSRRCI",
+					"CSRSI": "CSRRSI", "CSRWI": "CSRRWI", "CSRCI": "CSRRCI",
+				}[mnem]
+			} else {
+				csrMnem = map[string]string{
+					"CSRS": "CSRRS", "CSRW": "CSRRW", "CSRC": "CSRRC",
+					"CSRSI": "CSRRSI", "CSRWI": "CSRRWI", "CSRCI": "CSRRCI",
+				}[mnem]
+			}
+			csrEnc = riscvCsrTable[csrMnem]
 		default:
 			// Either src, csr, rd or csr, src, rd: a CSR *name* in the
 			// second operand marks the toolchain's order.
@@ -1541,6 +1556,17 @@ func encodeRISCVInstr(instr *ast.Instr, pc int, offsets map[string]int, fi riscv
 					csrOp, srcOp = ops[1], ops[0]
 				}
 			}
+			// An immediate source selects the immediate opcode (CSRRW $2,
+			// c, rd encodes CSRRWI, byte-identical to the explicit form),
+			// mirroring the toolchain's constant rewrite.
+			if isImmOperand(srcOp) {
+				csrMnem = map[string]string{
+					"CSRRW": "CSRRWI", "CSRRS": "CSRRSI", "CSRRC": "CSRRCI",
+				}[csrMnem]
+				if csrMnem != "" {
+					csrEnc = riscvCsrTable[csrMnem]
+				}
+			}
 		}
 		csr, ok := riscvCSRAddress(csrOp)
 		if !ok && csrFix == 0 {