Allow Polly at all optimization levels

Restricting Polly to -O3 does not make a lot of sense as it is opt-in anyway
and users who specifically request it should get it. If this causes performance
problems we should rather address them by scheduling the right cleanup passes
then just prevent the user from trying.

Also restricting Polly to -O3 made bugpoint not work with the -O3 flag and polly
enabled.

llvm-svn: 200208