Fix --help for prepass (default on)

driver/Driver.ml

@@ -207,7 +207,7 @@ Processing options:
   -fmove-loop-invariants Perform loop-invariant code motion [off]
   -fredundancy   Perform redundancy elimination [on]
   -mtune=         Type of CPU (for scheduling on some architectures)
-  -fprepass      Perform prepass scheduling (only on some architectures) [off]
+  -fprepass      Perform prepass scheduling (only on some architectures) [on]
   -fprepass= <optim> Perform postpass scheduling with the specified optimization [list]
                    (<optim>=list: list scheduling, <optim>=ilp: ILP, <optim>=greedy: just packing bundles)
   -fpostpass     Perform postpass scheduling (only for K1 architecture) [on]