Avoid wasteful array creation in backend

The iteration is only needed to side effect on `instructionList`
and `code.touched`. This commit uses an iterator and `foreach`,
rather than creating throwaway Arrays.

    % time (for f in {1..200}; do echo test/files/pos/t8893.scala; done | qscalac -Xresident)

    Before: 30s
    After : 21s

1 files changed, 10 insertions, 8 deletions

diff --git a/src/compiler/scala/tools/nsc/backend/icode/BasicBlocks.scala b/src/compiler/scala/tools/nsc/backend/icode/BasicBlocks.scala
index f9551697d2..ad1975ef23 100644
--- a/src/compiler/scala/tools/nsc/backend/icode/BasicBlocks.scala
+++ b/src/compiler/scala/tools/nsc/backend/icode/BasicBlocks.scala
@@ -300,14 +300,16 @@ trait BasicBlocks {
       if (!closed)
         instructionList = instructionList map (x => map.getOrElse(x, x))
       else
-        instrs.zipWithIndex collect {
-          case (oldInstr, i) if map contains oldInstr =>
-            // SI-6288 clone important here because `replaceInstruction` assigns
-            // a position to `newInstr`. Without this, a single instruction can
-            // be added twice, and the position last position assigned clobbers
-            // all previous positions in other usages.
-            val newInstr = map(oldInstr).clone()
-            code.touched |= replaceInstruction(i, newInstr)
+        instrs.iterator.zipWithIndex foreach {
+          case (oldInstr, i) =>
+            if (map contains oldInstr) {
+              // SI-6288 clone important here because `replaceInstruction` assigns
+              // a position to `newInstr`. Without this, a single instruction can
+              // be added twice, and the position last position assigned clobbers
+              // all previous positions in other usages.
+              val newInstr = map(oldInstr).clone()
+              code.touched |= replaceInstruction(i, newInstr)
+            }
         }
 
     ////////////////////// Emit //////////////////////