Also test in optimized mode

Author: aschwaighofer

test/embedded/existential.swift

@@ -1,5 +1,6 @@
 // RUN: %target-swift-frontend -enable-experimental-feature EmbeddedExistentials -enable-experimental-feature Embedded -parse-as-library -wmo -emit-sil %s | %FileCheck %s
 // RUN: %target-run-simple-swift(-enable-experimental-feature EmbeddedExistentials -enable-experimental-feature Embedded -parse-as-library -wmo) | %FileCheck %s --check-prefix=OUTPUT
+// RUN: %target-run-simple-swift(-enable-experimental-feature EmbeddedExistentials -enable-experimental-feature Embedded -parse-as-library -wmo -O) | %FileCheck %s --check-prefix=OUTPUT
 
 // REQUIRES: swift_in_compiler
 // REQUIRES: executable_test