[WebAssembly] Disable register stackification and coloring when not optimizing

These passes are optimizations, and should be disabled when not
optimizing.
Also create an MCCodeGenInfo so the opt level is correctly plumbed to
the backend pass manager.
Also remove the command line flag for disabling register coloring;
running llc with -O0 should now be useful for debugging, so it's not
necessary.

Differential Revision: http://reviews.llvm.org/D17327

llvm-svn: 261176