Disable Subzero IR generation for performance testing.

This CL allows one to time Subzero's bitcode parsing without IR generation
(other than types, function declarations, and uninitialized global variables)
for performance testing.

BUG=None
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/696383004

src/IceClFlags.h

diff --git a/src/IceClFlags.h b/src/IceClFlags.h
index 52b8f34..a1dcf68 100644
--- a/src/IceClFlags.h
+++ b/src/IceClFlags.h
@@ -26,8 +26,9 @@
         UseIntegratedAssembler(false), UseSandboxing(false),
         PhiEdgeSplit(false), DecorateAsm(false), DumpStats(false),
         AllowUninitializedGlobals(false), TimeEachFunction(false),
-        DefaultGlobalPrefix(""), DefaultFunctionPrefix(""), TimingFocusOn(""),
-        VerboseFocusOn(""), TranslateOnly("") {}
+        DisableIRGeneration(false), DefaultGlobalPrefix(""),
+        DefaultFunctionPrefix(""), TimingFocusOn(""), VerboseFocusOn(""),
+        TranslateOnly("") {}
   bool DisableInternal;
   bool SubzeroTimingEnabled;
   bool DisableTranslation;
@@ -40,6 +41,7 @@
   bool DumpStats;
   bool AllowUninitializedGlobals;
   bool TimeEachFunction;
+  bool DisableIRGeneration;
   IceString DefaultGlobalPrefix;
   IceString DefaultFunctionPrefix;
   IceString TimingFocusOn;