Disable Subzero IR generation for performance testing.

src/IceCfg.cpp

Index: src/IceCfg.cpp
diff --git a/src/IceCfg.cpp b/src/IceCfg.cpp
index bd1894ca72e20d2a029016026b538959940f14de..8e73499c46a8161c974510a1d621c382f8cef2ad 100644
--- a/src/IceCfg.cpp
+++ b/src/IceCfg.cpp
@@ -31,7 +31,10 @@ Cfg::Cfg(GlobalContext *Ctx)
       VMetadata(new VariablesMetadata(this)),
       TargetAssembler(
           TargetLowering::createAssembler(Ctx->getTargetArch(), this)),
-      CurrentNode(NULL) {}
+      CurrentNode(NULL) {
+  assert(!Ctx->isIRGenerationDisabled() &&
+         "Attempt to build cfg when IR generation disabled");
+}
 
 Cfg::~Cfg() {}