ART: Use core image to speed up some gtests.

Host timing of
  art_compiler_tests --no_isolate --gtest_filter='<pattern>'
for different patterns:
  "ms total"                    before after
  LoadStoreEliminationTest*      16945  4750
  LoadStoreAnalysisTest*          2647   689
  ReferenceTypePropagationTest*  13542  3929
  InstructionSimplifierTest*      1452   406

Host timing of
  art_runtime_tests --no_isolate --gtest_filter='<pattern>'
for different patterns:
  "ms total"                    before after
  RegType*Test*                   2976  1675
  DexCacheTest*                    265    74
  JavaVmExtTest*                   785   230

Host timing of
  art_libartbase_tests --no_isolate --gtest_filter='<pattern>'
for different patterns:
  "ms total"                    before after
  FlagsTests*                      691   214

Host timing of
  art_dex2oat_tests --no_isolate --gtest_filter='<pattern>'
for different patterns:
  "ms total"                    before after
  VerifierDepsTest*               3567   874

Test: m test-art-host-gtest
Change-Id: I20df90e3d38aaa286e22ba070c7845bcb09e3bca
diff --git a/compiler/optimizing/instruction_simplifier_test.cc b/compiler/optimizing/instruction_simplifier_test.cc
index 2063eed..c7c5b12 100644
--- a/compiler/optimizing/instruction_simplifier_test.cc
+++ b/compiler/optimizing/instruction_simplifier_test.cc
@@ -36,6 +36,10 @@
 template<typename SuperClass>
 class InstructionSimplifierTestBase : public SuperClass, public OptimizingUnitTestHelper {
  public:
+  InstructionSimplifierTestBase() {
+    this->use_boot_image_ = true;  // Make the Runtime creation cheaper.
+  }
+
   void SetUp() override {
     SuperClass::SetUp();
     gLogVerbosity.compiler = true;