Add an llvm option to enable/disable running the global value numbering
optimization pass.  Disabling this pass helps to minimize the size of
boot2.

Introduced here: http://svnweb.freebsd.org/changeset/base/274968

Index: lib/Transforms/IPO/PassManagerBuilder.cpp
===================================================================
--- lib/Transforms/IPO/PassManagerBuilder.cpp
+++ lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -78,6 +78,10 @@ static cl::opt<bool>
 EnableMLSM("mlsm", cl::init(true), cl::Hidden,
            cl::desc("Enable motion of merged load and store"));
 
+static cl::opt<bool> EnableGVN("enable-gvn",
+  cl::init(true), cl::Hidden,
+  cl::desc("Run the global value numbering pass"));
+
 PassManagerBuilder::PassManagerBuilder() {
     OptLevel = 2;
     SizeLevel = 0;
@@ -244,7 +248,8 @@ void PassManagerBuilder::populateModulePassManager
   if (OptLevel > 1) {
     if (EnableMLSM)
       MPM.add(createMergedLoadStoreMotionPass()); // Merge ld/st in diamonds
-    MPM.add(createGVNPass(DisableGVNLoadPRE));  // Remove redundancies
+    if (EnableGVN)
+      MPM.add(createGVNPass(DisableGVNLoadPRE));  // Remove redundancies
   }
   MPM.add(createMemCpyOptPass());             // Remove memcpy / form memset
   MPM.add(createSCCPPass());                  // Constant prop with SCCP