JavaMirrors: avoid the use of List flatten and map

In the JavaMirrors trait, in the function "mkMethodMirror",
the code was creating several unnecessary lists.

- We replace a call to `List.flatten.length` with a call to the
  `sumSize` method from the collections utils.
- We replace a `map` followed by an `exists` by merging the
  function of the `map` into the `exists`.
- We replace a call to `flatten` followed by an `exists`
  by a call to `mexists`.

This should, in total, prevent the creation of 5*N allocations,
with N being the sum of the length of all the tparams.

src/reflect/scala/reflect/runtime/JavaMirrors.scala

@@ -326,21 +326,20 @@ private[scala] trait JavaMirrors extends internal.SymbolTable with api.JavaUnive
     // that's because we want to have decent performance
     // therefore we move special cases into separate subclasses
     // rather than have them on a hot path them in a unified implementation of the `apply` method
-    private def mkMethodMirror[T: ClassTag](receiver: T, symbol: MethodSymbol): MethodMirror = {
-      def existsParam(pred: Type => Boolean) = symbol.paramss.flatten.map(_.info).exists(pred)
-      if (isBytecodelessMethod(symbol)) new BytecodelessMethodMirror(receiver, symbol)
-      else if (existsParam(isByNameParam) || existsParam(isValueClassParam)) new JavaTransformingMethodMirror(receiver, symbol)
-      else {
-        symbol.paramss.flatten.length match {
+    private def mkMethodMirror[T: ClassTag](receiver: T, symbol: MethodSymbol): MethodMirror =
+      if (isBytecodelessMethod(symbol))
+        new BytecodelessMethodMirror(receiver, symbol)
+      else if (mexists(symbol.paramss)(p => isByNameParam(p.info) || isValueClassParam(p.info)))
+        new JavaTransformingMethodMirror(receiver, symbol)
+      else
+        sumSize(symbol.paramss, 0) match {
           case 0 => new JavaVanillaMethodMirror0(receiver, symbol)
           case 1 => new JavaVanillaMethodMirror1(receiver, symbol)
           case 2 => new JavaVanillaMethodMirror2(receiver, symbol)
           case 3 => new JavaVanillaMethodMirror3(receiver, symbol)
           case 4 => new JavaVanillaMethodMirror4(receiver, symbol)
           case _ => new JavaVanillaMethodMirror(receiver, symbol)
         }
-      }
-    }
 
     private abstract class JavaMethodMirror(val symbol: MethodSymbol, protected val ret: DerivedValueClassMetadata) extends MethodMirror {
       lazy val jmeth = ensureAccessible(methodToJava(symbol))