Remove single-root assertion from Spec.root

- Assertion would search for root through all possible paths.
  - It's also really slow.

- This isn't needed anymore. We're pretty good at ensuring single-rooted
  DAGs, and this assertion has never been thrown.

- This shaves another 6 seconds off r-rminer concretization

lib/spack/spack/concretize.py

@@ -113,7 +113,9 @@ def choose_virtual_or_external(self, spec):
 
         # Find the nearest spec in the dag that has a compiler.  We'll
         # use that spec to calibrate compiler compatibility.
-        abi_exemplar = find_spec(spec, lambda x: x.compiler, spec.root)
+        abi_exemplar = find_spec(spec, lambda x: x.compiler)
+        if abi_exemplar is None:
+            abi_exemplar = spec.root
 
         # Sort candidates from most to least compatibility.
         #   We reverse because True > False.

lib/spack/spack/spec.py

@@ -1178,20 +1178,13 @@ def fullname(self):
     @property
     def root(self):
         """Follow dependent links and find the root of this spec's DAG.
-           In spack specs, there should be a single root (the package being
-           installed).  This will throw an assertion error if that is not
-           the case.
+
+        Spack specs have a single root (the package being installed).
         """
         if not self._dependents:
             return self
 
-        # If the spec has multiple dependents, ensure that they all
-        # lead to the same place.  Spack shouldn't deal with any DAGs
-        # with multiple roots, so something's wrong if we find one.
-        depiter = iter(self._dependents.values())
-        first_root = next(depiter).parent.root
-        assert(all(first_root is d.parent.root for d in depiter))
-        return first_root
+        return next(iter(self._dependents.values())).parent.root
 
     @property
     def package(self):