Fixes #980: really slow spack spec performance

- `spack spec dealii` now takse seconds rather than 18 minutes.

- Slow hash (`dag_hash`) was accidentally inserted into the fast hash
  (`__hash__`).

- Do not put the slow hash in the fast hash.

lib/spack/spack/spec.py

@@ -1750,8 +1750,7 @@ def _cmp_node(self):
                 self.variants,
                 self.architecture,
                 self.compiler,
-                self.compiler_flags,
-                self.dag_hash())
+                self.compiler_flags)
 
     def eq_node(self, other):
         """Equality with another spec, not including dependencies."""