Use malloc() to get space for task arguments when compiling to AVX.

This is to work around the LLVM bug/limitation discused in LLVM bug
10841 (http://llvm.org/bugs/show_bug.cgi?id=10841).

opt.cpp

@@ -187,6 +187,10 @@ Optimize(llvm::Module *module, int optLevel) {
     optPM.add(targetLibraryInfo);
     optPM.add(new llvm::TargetData(module));
 
+#if defined(LLVM_3_0) || defined(LLVM_3_0svn)
+    optPM.add(llvm::createIndVarSimplifyPass());
+#endif
+
     if (optLevel == 0) {
         // This is more or less the minimum set of optimizations that we
         // need to do to generate code that will actually run.  (We can't