[CanonicalizeOSSALifetime] Fixed caching bug.

Previously, the destroys set (now set vector) wasn't ever being cleared.
The result was that users could get overly pessimistic behavior if they
had previously used the utility with a destroy that came after the
destroys relevant for its current run. Here, it is cleared when the
utility is initialized with a new def.  Addresses a TODO in the
copy_propagation test.

Author: nate-chandler

include/swift/SILOptimizer/Utils/CanonicalizeOSSALifetime.h

@@ -304,6 +304,7 @@ class CanonicalizeOSSALifetime {
     // analysis, freeing its memory.
     accessBlocks = nullptr;
     consumes.clear();
+    destroys.clear();
 
     liveness.initializeDef(def);
   }

test/SILOptimizer/copy_propagation.sil

@@ -668,27 +668,13 @@ bb1:
 
 // Original Overlapping (unnecessarily extends pruned liveness):
 //
-// TODO: this copy could be avoided but is probably an unusual case,
-// and sinking the destroy outside the access scope might help to
-// optimize the access itself.
-//
-//     %def
-//     begin_access // access scope unrelated to def
-//     use %def     // pruned liveness ends here
-//     br bb1
-// bb1:
-//     destroy %def
-//     end_access
-//
 // CHECK-LABEL: sil [ossa] @testOriginalOverlapInDeadBlock : $@convention(thin) () -> () {
 // CHECK:   [[DEF:%.*]] = apply %{{.*}}() : $@convention(thin) () -> @owned AnyObject
 // CHECK:   begin_access
-// CHECK:   [[COPY:%.*]] = copy_value [[DEF]] : $AnyObject
-// CHECK:   store [[COPY]] to [init] %{{.*}} : $*AnyObject
+// CHECK:   store [[DEF]] to [init] %{{.*}} : $*AnyObject
 // CHECK:   br bb1
 // CHECK: bb1:
 // CHECK:   end_access
-// CHECK:   destroy_value [[DEF]] : $AnyObject
 // CHECK-LABEL: } // end sil function 'testOriginalOverlapInDeadBlock'
 sil [ossa] @testOriginalOverlapInDeadBlock : $@convention(thin) () -> () {
 bb0: