Fix Clang error when disabling MOP FUSION

When using 'make ENABLE_MOP_FUSION=0 CC=clang', the following error is
observed:
src/emulate.c:695:20: error: unused function 'remove_next_nth_ir' [-Werror,-Wunused-function]
  695 | static inline void remove_next_nth_ir(const riscv_t *rv,
	  |                    ^~~~~~~~~~~~~~~~~~
1 error generated.

Fix by gurading the 'remove_next_nth_ir' function and the
'COMBINE_MEM_OPS' macro with the 'RV32_HAS(MOP_FUSION)' macro. This
ensures both are only included when MOP FUSION is enabled, preventing the
Clang warning and maintaining proper functionality.

Author: eleanorLYJ

src/emulate.c

@@ -666,6 +666,7 @@ static void block_translate(riscv_t *rv, block_t *block)
     block->ir_tail->next = NULL;
 }
 
+#if RV32_HAS(MOP_FUSION)
 #define COMBINE_MEM_OPS(RW)                                       \
     next_ir = ir->next;                                           \
     count = 1;                                                    \
@@ -705,7 +706,6 @@ static inline void remove_next_nth_ir(const riscv_t *rv,
     block->n_insn -= n;
 }
 
-#if RV32_HAS(MOP_FUSION)
 /* Check if instructions in a block match a specific pattern. If they do,
  * rewrite them as fused instructions.
  *