[region-isolation] Eliminate a temporary std::vector.

We can just pass in an ArrayRef here. I missed this inefficiency when I was
going through the pass earlier.

Author: gottesmm

include/swift/SILOptimizer/Utils/PartitionUtils.h

@@ -322,7 +322,7 @@ class Partition {
   // so set to false to begin with
   Partition(bool canonical) : labels({}), canonical(canonical) {}
 
-  static Partition singleRegion(std::vector<Element> indices) {
+  static Partition singleRegion(ArrayRef<Element> indices) {
     Partition p;
     if (!indices.empty()) {
       Region min_index =