[DCE] Insts that define lexical values seem useful

Don't delete as dead instructions that define lexical values such as
`move_value [lexical]`.

rdar://129299803

Author: nate-chandler

lib/SILOptimizer/Transforms/DeadCodeElimination.cpp

@@ -63,6 +63,11 @@ static bool seemsUseful(SILInstruction *I) {
   if (I->mayHaveSideEffects())
     return true;
 
+  if (llvm::any_of(I->getResults(),
+                   [](auto result) { return result->isLexical(); })) {
+    return true;
+  }
+
   if (auto *BI = dyn_cast<BuiltinInst>(I)) {
     // Although the onFastPath builtin has no side-effects we don't want to
     // remove it.

test/SILOptimizer/dead_code_elimination_ossa.sil

@@ -13,6 +13,9 @@ typealias Int1 = Builtin.Int1
 
 class C {}
 
+sil @getC : $@convention(thin) () -> @owned C
+sil @barrier : $@convention(thin) () -> ()
+
 struct CAndBit {
     var c: C
     var bit: Int1
@@ -468,3 +471,20 @@ bb3:
   %22 = tuple ()
   return %22 : $()
 }
+
+// CHECK-LABEL: sil [ossa] @dont_delete_move_value_lexical : {{.*}} {
+// CHECK:         [[LEXICAL:%[^,]+]] = move_value [lexical]
+// CHECK:         [[DUMMY:%[^,]+]] = function_ref @dummy
+// CHECK:         apply [[DUMMY]]()
+// CHECK:         destroy_value [[LEXICAL]]
+// CHECK-LABEL: } // end sil function 'dont_delete_move_value_lexical'
+sil [ossa] @dont_delete_move_value_lexical : $@convention(thin) () -> () {
+  %getC = function_ref @getC : $@convention(thin) () -> @owned C
+  %c = apply %getC() : $@convention(thin) () -> @owned C
+  %m = move_value [lexical] %c : $C
+  %dummy = function_ref @dummy : $@convention(thin) () -> ()
+  apply %dummy() : $@convention(thin) () -> ()
+  destroy_value %m : $C
+  %retval = tuple ()
+  return %retval : $()
+}