remove DefRegInit, change DefReg API with option definition.

core/src/main/scala/chisel3/Reg.scala

@@ -42,7 +42,7 @@ object Reg {
     val clock = Node(Builder.forcedClock)
 
     reg.bind(RegBinding(Builder.forcedUserModule, Builder.currentWhen()))
-    pushCommand(DefReg(sourceInfo, reg, clock))
+    pushCommand(DefReg(sourceInfo, reg, clock, None))
     reg
   }
 
@@ -176,7 +176,7 @@ object RegInit {
 
     reg.bind(RegBinding(Builder.forcedUserModule, Builder.currentWhen()))
     requireIsHardware(init, "reg initializer")
-    pushCommand(DefRegInit(sourceInfo, reg, clock.ref, reset.ref, init.ref))
+    pushCommand(DefReg(sourceInfo, reg, clock.ref, Some(RegInitIR(reset.ref, init.ref))))
     reg
   }
 

core/src/main/scala/chisel3/internal/firrtl/Converter.scala

@@ -117,10 +117,10 @@ private[chisel3] object Converter {
       Some(fir.DefNode(convert(e.sourceInfo), e.name, expr))
     case e @ DefWire(info, id) =>
       Some(fir.DefWire(convert(info), e.name, extractType(id, info)))
-    case e @ DefReg(info, id, clock) =>
+    case e @ DefReg(info, id, clock, None) =>
       Some(fir.DefRegister(convert(info), e.name, extractType(id, info), convert(clock, ctx, info),
                            firrtl.Utils.zero, convert(getRef(id, info), ctx, info)))
-    case e @ DefRegInit(info, id, clock, reset, init) =>
+    case e @ DefReg(info, id, clock, Some(RegInitIR(reset, init))) =>
       Some(fir.DefRegister(convert(info), e.name, extractType(id, info), convert(clock, ctx, info),
                            convert(reset, ctx, info), convert(init, ctx, info)))
     case e @ DefMemory(info, id, t, size) =>

core/src/main/scala/chisel3/internal/firrtl/IR.scala

@@ -758,8 +758,8 @@ abstract class Definition extends Command {
 case class DefPrim[T <: Data](sourceInfo: SourceInfo, id: T, op: PrimOp, args: Arg*) extends Definition
 case class DefInvalid(sourceInfo: SourceInfo, arg: Arg) extends Command
 case class DefWire(sourceInfo: SourceInfo, id: Data) extends Definition
-case class DefReg(sourceInfo: SourceInfo, id: Data, clock: Arg) extends Definition
-case class DefRegInit(sourceInfo: SourceInfo, id: Data, clock: Arg, reset: Arg, init: Arg) extends Definition
+case class RegInitIR(reset: Arg, init: Arg)
+case class DefReg(sourceInfo: SourceInfo, id: Data, clock: Arg, regInit: Option[RegInitIR]) extends Definition
 case class DefMemory(sourceInfo: SourceInfo, id: HasId, t: Data, size: BigInt) extends Definition
 case class DefSeqMemory(sourceInfo: SourceInfo, id: HasId, t: Data, size: BigInt, readUnderWrite: fir.ReadUnderWrite.Value) extends Definition
 case class DefMemPort[T <: Data](sourceInfo: SourceInfo, id: T, source: Node, dir: MemPortDirection, index: Arg, clock: Arg) extends Definition

src/main/scala/chisel3/aop/Select.scala

@@ -101,7 +101,6 @@ object Select {
     check(module)
     module._component.get.asInstanceOf[DefModule].commands.collect {
       case r: DefReg => r.id
-      case r: DefRegInit => r.id
     }
   }
 

src/main/scala/chisel3/internal/firrtl/Emitter.scala

@@ -66,8 +66,7 @@ private class Emitter(circuit: Circuit) {
     val firrtlLine = e match {
       case e: DefPrim[_] => s"node ${e.name} = ${e.op.name}(${e.args.map(_.fullName(ctx)).mkString(", ")})"
       case e: DefWire => s"wire ${e.name} : ${emitType(e.id)}"
-      case e: DefReg => s"reg ${e.name} : ${emitType(e.id)}, ${e.clock.fullName(ctx)}"
-      case e: DefRegInit => s"reg ${e.name} : ${emitType(e.id)}, ${e.clock.fullName(ctx)} with : (reset => (${e.reset.fullName(ctx)}, ${e.init.fullName(ctx)}))"
+      case e: DefReg => s"reg ${e.name} : ${emitType(e.id)}, ${e.clock.fullName(ctx)}${ if (e.regInit.isDefined) "with : (reset => (${e.reset.fullName(ctx)}, ${e.init.fullName(ctx)}))" else ""}"
       case e: DefMemory => s"cmem ${e.name} : ${emitType(e.t)}[${e.size}]"
       case e: DefSeqMemory => s"smem ${e.name} : ${emitType(e.t)}[${e.size}], ${e.readUnderWrite}"
       case e: DefMemPort[_] => s"${e.dir} mport ${e.name} = ${e.source.fullName(ctx)}[${e.index.fullName(ctx)}], ${e.clock.fullName(ctx)}"