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
@@ -57,6 +57,10 @@ static cl::opt<bool> RunLoadCombine("combine-loads
                                     cl::Hidden,
                                     cl::desc("Run the load combining pass"));
 
+static cl::opt<bool> RunGVN("enable-gvn", cl::init(true),
+                            cl::Hidden,
+                            cl::desc("Run the global value numbering pass"));
+
 PassManagerBuilder::PassManagerBuilder() {
     OptLevel = 2;
     SizeLevel = 0;
@@ -210,7 +214,8 @@ void PassManagerBuilder::populateModulePassManager
 
   if (OptLevel > 1) {
     MPM.add(createMergedLoadStoreMotionPass()); // Merge load/stores in diamond
-    MPM.add(createGVNPass());                 // Remove redundancies
+    if (RunGVN)
+      MPM.add(createGVNPass());                 // Remove redundancies
   }
   MPM.add(createMemCpyOptPass());             // Remove memcpy / form memset
   MPM.add(createSCCPPass());                  // Constant prop with SCCP