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 ab9d5f724e826babb9e60a9315faf5928ebd5a7c..c04a12933f3ab2c047d8e10e616c9e047273e58b 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->getFlags().DisableIRGeneration &&
+         "Attempt to build cfg when IR generation disabled");
+}
 
 Cfg::~Cfg() {}