Simplify OP_trunc byte truncation in ARM and RISC-V codegen

- Remove unnecessary intermediate variable assignment in byte truncation

Author: hhhh1300

src/arm-codegen.c

@@ -440,8 +440,7 @@ void emit_ph2_ir(ph2_ir_t *ph2_ir)
         return;
     case OP_trunc:
         if (rm == 1) {
-            rm = 0xFF;
-            emit(__and_i(__AL, rd, rn, rm));
+            emit(__and_i(__AL, rd, rn, 0xFF));
         } else if (rm == 2) {
             emit(__sll_amt(__AL, 0, logic_ls, rd, rn, 16));
             emit(__sll_amt(__AL, 0, logic_rs, rd, rd, 16));

src/riscv-codegen.c

@@ -431,8 +431,7 @@ void emit_ph2_ir(ph2_ir_t *ph2_ir)
         return;
     case OP_trunc:
         if (ph2_ir->src1 == 1) {
-            rs2 = 0xFF;
-            emit(__andi(rd, rs1, rs2));
+            emit(__andi(rd, rs1, 0xFF));
         } else if (ph2_ir->src1 == 2) {
             /* For short truncation,
              * use shift operations since 0xFFFF is too large