Fix the no-swift-assert stdlib build. (#19037)

Partially revert a recent attempt to fix build warnings.

Fixes: <rdar://problem/43824067> Swift CI:
1. OSS - Swift (Tools Opt+No Assert, Stdlib Opt+DebInfo, Test Simulator)
   - OS X (master) - SimplifyCFG crashes at AssertCommon.swift:283:10

Author: atrick

lib/SILOptimizer/Transforms/SpeculativeDevirtualizer.cpp

@@ -218,8 +218,11 @@ static FullApplySite speculateMonomorphicTarget(FullApplySite AI,
   NumTargetsPredicted++;
 
   // Devirtualize the apply instruction on the identical path.
-  assert(devirtualizeClassMethod(IdenAI, DownCastedClassInstance, nullptr) &&
-         "Expected to be able to devirtualize apply!");
+  auto NewInst =
+    devirtualizeClassMethod(IdenAI, DownCastedClassInstance, nullptr);
+  assert(NewInst && "Expected to be able to devirtualize apply!");
+  (void)NewInst;
+
   deleteDevirtualizedApply(IdenAI);
 
   // Split critical edges resulting from VirtAI.