Optimizer: add an additional DeadObjectElimination at the end of the pipeline

The last dead-store-elimination pass can expose opportunities for dead object elimination.

rdar://110846405

Author: eeckstein

lib/SILOptimizer/PassManager/PassPipeline.cpp

@@ -760,6 +760,9 @@ static void addLowLevelPassPipeline(SILPassPipelinePlan &P) {
   P.addObjectOutliner();
   P.addDeadStoreElimination();
 
+  // dead-store-elimination can expose opportunities for dead object elimination.
+  P.addDeadObjectElimination();
+
   // We've done a lot of optimizations on this function, attempt to FSO.
   P.addFunctionSignatureOpts();
   P.addComputeEscapeEffects();

test/SILOptimizer/dead_alloc_stack.swift renamed to test/SILOptimizer/dead_alloc.swift

@@ -19,14 +19,26 @@ func g<T : P>(_ x : T) -> Bool {
 
 // Check that this function can be completely constant folded and no alloc_stack remains.
 
-// CHECK-LABEL: sil @$s16dead_alloc_stack6testitySbAA1XVF
+// CHECK-LABEL: sil @$s10dead_alloc0A10AllocStackySbAA1XVF :
 // CHECK:      bb0({{.*}}):
 // CHECK-NEXT:   debug_value
 // CHECK-NEXT:   integer_literal
 // CHECK-NEXT:   struct
 // CHECK-NEXT:   return
-// CHECK-NEXT: } // end sil function '$s16dead_alloc_stack6testitySbAA1XVF'
-public func testit(_ x: X) -> Bool {
+// CHECK-NEXT: } // end sil function '$s10dead_alloc0A10AllocStackySbAA1XVF'
+public func deadAllocStack(_ x: X) -> Bool {
   return g(x)
 }
 
+public class C<T> {
+    let x: String = "123"
+}
+
+// CHECK-LABEL: sil @$s10dead_alloc0A13ClassInstanceyyF :
+// CHECK:       bb0:
+// CHECK-NEXT:    tuple
+// CHECK-NEXT:    return
+// CHECK-NEXT:  } // end sil function '$s10dead_alloc0A13ClassInstanceyyF'
+public func deadClassInstance() {
+    let _ = C<Int>()
+}