New flag: -ddump-simpl-phases

This outputs the core after each simplifier phase (i.e., it produces less
information that -ddump-simpl-iterations).

compiler/main/DynFlags.hs

@@ -126,6 +126,7 @@ data DynFlag
    | Opt_D_dump_rn
    | Opt_D_dump_simpl
    | Opt_D_dump_simpl_iterations
+   | Opt_D_dump_simpl_phases
    | Opt_D_dump_spec
    | Opt_D_dump_prep
    | Opt_D_dump_stg
@@ -1079,6 +1080,7 @@ dynamic_flags = [
   ,  ( "ddump-rn",           	 setDumpFlag Opt_D_dump_rn)
   ,  ( "ddump-simpl",        	 setDumpFlag Opt_D_dump_simpl)
   ,  ( "ddump-simpl-iterations", setDumpFlag Opt_D_dump_simpl_iterations)
+  ,  ( "ddump-simpl-phases",     setDumpFlag Opt_D_dump_simpl_phases)
   ,  ( "ddump-spec",         	 setDumpFlag Opt_D_dump_spec)
   ,  ( "ddump-prep",          	 setDumpFlag Opt_D_dump_prep)
   ,  ( "ddump-stg",          	 setDumpFlag Opt_D_dump_stg)

compiler/simplCore/SimplCore.lhs

@@ -407,7 +407,7 @@ simplifyPgm mode switches hsc_env us imp_rule_base guts
 			 text "",
 			 pprSimplCount counts_out]);
 
-	endPass dflags ("Simplify phase " ++ phase_info ++ " done") Opt_D_verbose_core2core binds';
+	endPass dflags ("Simplify phase " ++ phase_info ++ " done") Opt_D_dump_simpl_phases binds';
 
 	return (counts_out, guts { mg_binds = binds' })
     }