backend: fix exe unit pipeline error

Add the Busy register in exe units to indicate that new instructions
cannot be received, but it does not allow the exe unit to be pipelinable.
Todo: make exe unit pipelinable.

src/main/scala/xiangshan/v2backend/exu/ExeUnit.scala

@@ -40,6 +40,16 @@ class ExeUnitImp(
     module
   })
 
+  val busy = RegInit(false.B)
+  val robIdx = RegEnable(io.in.bits.robIdx, io.in.fire)
+  when (robIdx.needFlush(io.flush)) {
+    busy := false.B
+  }.elsewhen(io.out.fire) {
+    busy := false.B
+  }.elsewhen(io.in.fire) {
+    busy := true.B
+  }
+
   // rob flush --> funcUnits
   funcUnits.zipWithIndex.foreach { case (fu, i) =>
     fu.io.flush <> io.flush
@@ -52,7 +62,9 @@ class ExeUnitImp(
   val in1ToN = Module(new Dispatcher(new ExuInput(exuParams), funcUnits.length, acceptCond))
 
   // ExeUnit.in <---> Dispatcher.in
-  in1ToN.io.in <> io.in
+  in1ToN.io.in.valid := io.in.valid && !busy
+  in1ToN.io.in.bits := io.in.bits
+  io.in.ready := !busy
 
   // Dispatcher.out <---> FunctionUnits
   in1ToN.io.out.zip(funcUnits.map(_.io.in)).foreach {