performance: don't recompute hashes when regenerating environments

`ViewDescriptor.regenerate()` was copying specs and stripping build
dependencies, which clears `_hash` and other cached fields on concrete
specs, which causes a bunch of YAML hashes to be recomputed.

- [x] Preserve the `_hash` and `_normal` fields on stripped specs, as
  these will be unchanged.

lib/spack/spack/environment.py

@@ -511,10 +511,15 @@ def regenerate(self, all_specs, roots):
         specs = all_specs if self.link == 'all' else roots
         for spec in specs:
             # The view does not store build deps, so if we want it to
-            # recognize environment specs (which do store build deps), then
-            # they need to be stripped
+            # recognize environment specs (which do store build deps),
+            # then they need to be stripped.
             if spec.concrete:  # Do not link unconcretized roots
-                specs_for_view.append(spec.copy(deps=('link', 'run')))
+                # We preserve _hash _normal to avoid recomputing DAG
+                # hashes (DAG hashes don't consider build deps)
+                spec_copy = spec.copy(deps=('link', 'run'))
+                spec_copy._hash = spec._hash
+                spec_copy._normal = spec._normal
+                specs_for_view.append(spec_copy)
 
         # regeneration queries the database quite a bit; this read
         # transaction ensures that we don't repeatedly lock/unlock.

lib/spack/spack/spec.py

@@ -2999,7 +2999,7 @@ def _dup(self, other, deps=True, cleardeps=True, caches=None):
                 before possibly copying the dependencies of ``other`` onto
                 ``self``
             caches (bool or None): preserve cached fields such as
-                ``_normal``, ``_concrete``, and ``_cmp_key_cache``. By
+                ``_normal``, ``_hash``, and ``_cmp_key_cache``. By
                 default this is ``False`` if DAG structure would be
                 changed by the copy, ``True`` if it's an exact copy.