Add groupByIntoSeq from firrtl utils

src/main/scala/chisel3/util/experimental/decode/TruthTable.scala

@@ -3,7 +3,7 @@
 package chisel3.util.experimental.decode
 
 import chisel3.util.BitPat
-import firrtl.Utils.groupByIntoSeq
+import scala.collection.mutable
 
 sealed class TruthTable private (val table: Seq[(BitPat, BitPat)], val default: BitPat, val sort: Boolean) {
   def inputWidth = table.head._1.getWidth
@@ -190,4 +190,15 @@ object TruthTable {
       bitPat(tables.flatMap { case (table, indexes) => table.default.rawString.zip(indexes) })
     )
   }
+
+  /** Similar to Seq.groupBy except that it preserves ordering of elements within each group */
+  private def groupByIntoSeq[A, K](xs: Iterable[A])(f: A => K): Seq[(K, Seq[A])] = {
+    val map = mutable.LinkedHashMap.empty[K, mutable.ListBuffer[A]]
+    for (x <- xs) {
+      val key = f(x)
+      val l = map.getOrElseUpdate(key, mutable.ListBuffer.empty[A])
+      l += x
+    }
+    map.view.map({ case (k, vs) => k -> vs.toList }).toList
+  }
 }