Add --opt=force-aligned-memory option.

This forces all vector loads/stores to be done assuming that the given
pointer is aligned to the vector size, thus allowing the use of sometimes
more-efficient instructions.  (If it isn't the case that the memory is
aligned, the program will fail!).

module.cpp

@@ -1783,6 +1783,8 @@ Module::execPreprocessor(const char* infilename, llvm::raw_string_ostream* ostre
         opts.addMacroDef("ISPC_TARGET_HAS_HALF");
     if (g->target.hasTranscendentals)
         opts.addMacroDef("ISPC_TARGET_HAS_TRANSCENDENTALS");
+    if (g->opt.forceAlignedMemory)
+        opts.addMacroDef("ISPC_FORCE_ALIGNED_MEMORY");
 
     opts.addMacroDef("ISPC_MAJOR_VERSION=1");
     opts.addMacroDef("ISPC_MINOR_VERSION=3");