EscapeAnalysis: fix a quadratic behavior in ConnectionGraph::getNode

Fixes a compile time problem. The single linked list of merge targets in connection graph nodes can be very large.
Update the final merge target in the map, so that it has to be traversed only once for a given SILValue.

rdar://problem/71602804

Author: eeckstein

lib/SILOptimizer/Analysis/EscapeAnalysis.cpp

@@ -450,6 +450,9 @@ EscapeAnalysis::ConnectionGraph::getNode(SILValue V) {
   if (Node) {
     CGNode *targetNode = Node->getMergeTarget();
     targetNode->mergeFlags(false /*isInterior*/, hasReferenceOnly);
+    // Update the node in Values2Nodes, so that next time we don't need to find
+    // the final merge target.
+    Node = targetNode;
     return targetNode;
   }
   if (isa<SILFunctionArgument>(ptrBase)) {

validation-test/SILOptimizer/large_nested_array.swift.gyb

@@ -0,0 +1,27 @@
+// RUN: %empty-directory(%t)
+// RUN: %gyb %s > %t/main.swift
+
+// The compiler should finish in less than 2 minautes. To give some slack,
+// specify a timeout of 4 minutes.
+// If the compiler needs more than 5 minutes, there is probably a real problem.
+// So please don't just increase the timeout in case this fails.
+
+// RUN: %{python} %S/../../test/Inputs/timeout.py 240 %target-swift-frontend -O -parse-as-library -sil-verify-none -c %t/main.swift -o %t/main.o
+
+// REQUIRES: swift_stdlib_no_asserts,optimized_stdlib
+// REQUIRES: long_test
+// REQUIRES: CPU=arm64 || CPU=x86_64
+
+public struct TestStruct {
+  public static var a: [[Int]] {
+    var a: [[Int]] = Array(repeating: Array(repeating: 0, count: 4), count: 2000)
+
+% for i in range(2000):
+    a[${i}] = [${i * 4}, ${i * 4 + 1}, ${i * 4 + 2}, ${i * 4 + 3}]
+% end
+
+    return a
+  }
+}
+
+