turn on memset idiom recognition by default. Though there are still lots of

limitations, this kicks in dozens of times in the 4 specfp2000 benchmarks, 
and hundreds of times in the int part.  It also kicks in hundreds of times 
in multisource.

This kicks in right before loop deletion, which has the pleasant effect of
deleting loops that *just* do a memset.

llvm-svn: 122664