Fix EscapeAnalysis losing precision during merge.

Array storage was being stack promoted even though it escaped. This
happened because multiple locally allocated arrays were merged into
the same locally allocated array value box. For this to become a
problem, other bizarre merge events need to take place, such as a
value node being mapped with a content node. The series of events led
to a missing edge in the connection graph. One of the arrays was
mapped directly to a project_box instruction which had forgotten it's
relationship with the alloc_box.

It is a trivial one line fix to simply preserve all value mappings.

<rdar://58371330> app crashes (miscompile)

Author: atrick

lib/SILOptimizer/Analysis/EscapeAnalysis.cpp

@@ -732,9 +732,13 @@ void EscapeAnalysis::ConnectionGraph::mergeAllScheduledNodes() {
     From->isMerged = true;
 
     if (From->mappedValue) {
-      if (To->mappedValue)
-        Values2Nodes.erase(From->mappedValue);
-      else {
+      // If possible, transfer 'From's mappedValue to 'To' for clarity. Any
+      // values previously mapped to 'From' but not transferred to 'To's
+      // mappedValue must remain mapped to 'From'. Lookups on those values will
+      // find 'To' via the mergeTarget. Dropping a value's mapping is illegal
+      // because it could cause a node to be recreated without the edges that
+      // have already been discovered.
+      if (!To->mappedValue) {
         To->mappedValue = From->mappedValue;
         Values2Nodes[To->mappedValue] = To;
       }

test/SILOptimizer/escape_analysis.sil

@@ -1817,47 +1817,3 @@ bb0(%0 : $IntWrapper):
   %tuple = tuple (%bridge : $Builtin.BridgeObject, %ump : $UnsafeMutablePointer<Int64>)
   return %tuple : $(Builtin.BridgeObject, UnsafeMutablePointer<Int64>)
 }
-
-// =============================================================================
-// Test call to array.uninitialized that has extra release_value uses
-
-class DummyArrayStorage<Element> {
-  @_hasStorage var count: Int { get }
-  @_hasStorage var capacity: Int { get }
-  init()
-}
-
-// init_any_array_with_buffer
-sil [_semantics "array.uninitialized"] @init_any_array_with_buffer : $@convention(thin) (@owned DummyArrayStorage<AnyObject>, Int32, @thin Array<AnyObject>.Type) -> (@owned Array<AnyObject>, UnsafeMutablePointer<AnyObject>)
-
-// CHECK-LABEL: CG of testBadArrayUninit
-// CHECK-NEXT:  Val [ref] %2 Esc: , Succ: (%2.1)
-// CHECK-NEXT:  Con [int] %2.1 Esc: G, Succ: (%2.2)
-// CHECK-NEXT:  Con [ref] %2.2 Esc: G, Succ:
-// CHECK-NEXT:  Val %5 Esc: , Succ: (%5.1)
-// CHECK-NEXT:  Con %5.1 Esc: G, Succ: %10
-// CHECK-NEXT:  Val [ref] %10 Esc: G, Succ: (%10.1)
-// CHECK-NEXT:  Con %10.1 Esc: G, Succ:
-// CHECK-LABEL: End
-sil hidden @testBadArrayUninit : $@convention(thin) (Builtin.Word, Int32) -> () {
-bb0(%0 : $Builtin.Word, %1 : $Int32):
-  // create an array
-  %2 = alloc_ref [tail_elems $AnyObject * %0 : $Builtin.Word] $DummyArrayStorage<AnyObject>
-  %3 = metatype $@thin Array<AnyObject>.Type
-  %4 = function_ref @init_any_array_with_buffer : $@convention(thin) (@owned DummyArrayStorage<AnyObject>, Int32, @thin Array<AnyObject>.Type) -> (@owned Array<AnyObject>, UnsafeMutablePointer<AnyObject>)
-  %5 = apply %4(%2, %1, %3) : $@convention(thin) (@owned DummyArrayStorage<AnyObject>, Int32, @thin Array<AnyObject>.Type) -> (@owned Array<AnyObject>, UnsafeMutablePointer<AnyObject>)
-  %6 = tuple_extract %5 : $(Array<AnyObject>, UnsafeMutablePointer<AnyObject>), 0
-  %7 = tuple_extract %5 : $(Array<AnyObject>, UnsafeMutablePointer<AnyObject>), 1
-  %8 = struct_extract %7 : $UnsafeMutablePointer<AnyObject>, #UnsafeMutablePointer._rawValue
-  %9 = pointer_to_address %8 : $Builtin.RawPointer to [strict] $*AnyObject
-
-  // store an elt
-  %10 = alloc_ref $C
-  %11 = init_existential_ref %10 : $C : $C, $AnyObject
-  store %11 to %9 : $*AnyObject
-
-  // extra use of the call
-  release_value %5 : $(Array<AnyObject>, UnsafeMutablePointer<AnyObject>) // id: %228
-  %13 = tuple ()
-  return %13 : $()
-}