Add HazardPessimisticPlugin for light and very good FMAX hazard tracking

src/main/scala/SpinalRiscv/Plugin/HazardPessimisticPlugin.scala

@@ -0,0 +1,17 @@
+package SpinalRiscv.Plugin
+
+import SpinalRiscv._
+import spinal.core._
+import spinal.lib._
+
+
+class HazardPessimisticPlugin() extends Plugin[VexRiscv] {
+  import Riscv._
+  override def build(pipeline: VexRiscv): Unit = {
+    import pipeline._
+    import pipeline.config._
+
+    val writesInPipeline = List(execute,memory,writeBack).map(s => s.arbitration.isValid && s.input(REGFILE_WRITE_VALID)) :+ RegNext(writeBack.arbitration.isValid && writeBack.input(REGFILE_WRITE_VALID))
+    decode.arbitration.haltIt.setWhen(decode.arbitration.isValid && writesInPipeline.orR)
+  }
+}