RISC-V: Fix epilogue generation for barrier.

 I noticed that I have made a mistake in previous patch:
 https://patchwork.sourceware.org/project/gcc/patch/20220817071950.271762-1-juzhe.zhong@rivai.ai/

 The previous statement before this patch:
 bool need_barrier_p = (get_frame_size () + cfun->machine->frame.arg_pointer_offset) != 0;

 However, I changed it in the previous patch:
 bool need_barrier_p = known_ne (get_frame_size (), cfun->machine->frame.arg_pointer_offset);
 This is incorrect.

 Now, I correct this statement in this patch.

gcc/ChangeLog:

	* config/riscv/riscv.cc (riscv_expand_epilogue): Fix statement.