SILOptimizer: Replace [].append(contentsOf:) with [].append(element:)

if the argument is an array literal.

For example:
  arr += [1, 2, 3]

is replaced by:
  arr.append(1)
  arr.append(2)
  arr.append(3)

This gives considerable speedups up to 10x (for our micro-benchmarks which test this).

This is based on the work of @ben-ng, who implemented the first version of this optimization (thanks!).

include/swift/SILOptimizer/Analysis/ArraySemantic.h

@@ -135,6 +135,12 @@ public:
   /// Returns true on success, false otherwise.
   bool replaceByValue(SILValue V);
 
+  /// Replace a call to append(contentsOf: ) with a series of
+  /// append(element: ) calls.
+  bool replaceByAppendingValues(SILModule &M, SILFunction *AppendFn,
+                                const llvm::SmallVectorImpl<SILValue> &Vals,
+                                ArrayRef<Substitution> Subs);
+
   /// Hoist the call to the insert point.
   void hoist(SILInstruction *InsertBefore, DominanceInfo *DT) {
     hoistOrCopy(InsertBefore, DT, false);