Do memread stalling correctly.

src/main/scala/IFBarrier.scala

@@ -10,11 +10,25 @@ class IFBarrier extends MultiIOModule {
       val PCout = Output(UInt(32.W))
       val instructionIn = Input(new Instruction)
       val instructionOut = Output(new Instruction)
+      val stall = Input(Bool())
     })
 
   val PC = RegInit(UInt(32.W), 0.U)
-  PC := io.PCin
+  PC := Mux(io.stall, PC, io.PCin)
   io.PCout := PC
 
-  io.instructionOut := io.instructionIn
+  val instruction = Reg(new Instruction)
+  val replay = RegInit(Bool(), false.B)
+  replay := io.stall
+  instruction := io.instructionIn
+
+  io.instructionOut := Mux(
+    io.stall,
+    Instruction.NOP,
+    Mux(
+      replay,
+      instruction,
+      io.instructionIn
+    )
+  )
 }