Add an enable_verifier setting.

This is off by default, but enabled by the parser when reading a textual
IL file. Test files can still override the default to turn off
verification.

The setting enables IL verifier passes at critical points of the
compilation pipeline.

lib/cretonne/meta/base/settings.py

@@ -18,6 +18,14 @@ opt_level = EnumSetting(
         """,
         'default', 'best', 'fastest')
 
+enable_verifier = BoolSetting(
+        """
+        Run the Cretonne IL verifier at strategic times during compilation.
+
+        This makes compilation slower but catches many bugs. The verifier is
+        disabled by default, except when reading Cretonne IL from a text file.
+        """)
+
 is_64bit = BoolSetting("Enable 64-bit code generation")
 
 is_compressed = BoolSetting("Enable compressed instructions")