[PATCH][LTO] Rework -flto-partition=, add =one case

[Jan Hubicka]

> 
> This reworks the option to use the Enum support we have now and
> adds a =one case (to eventually get rid of one LTO operation mode,
> =none ...).  I was tempted to support -flto-partition=<number>
> and get rid of --param lto-partitions (thereby also supporting =1),

Yep, I preffer to have one switch to chose algorithm and other to set
its parameter as you do now. At the moment partitioning is quite a non-issue
since only important IPA passes works on whole thing, but that may change and
we may want to play with different partitionings.
(I have plans for that for incremental compilation and other things)

> but that param specifies the maximum number of partitions and
> still uses the balanced algorithm, thus the result would be
> confusing (and of little use I suppose, as opposed to =1 which should
> give you the same answer as =none).

=none still seems somewhat useful - for setups where you do multiple parallel
compilations it will be faster than WHOPR and it helps developing IPA passes
since you do not need to worry about WHOPR complexities at start.

But with the code to bring function bodies at demand, this is less important.
I believe with passmanager being bit more flexible, the code paths can be
almost completely shared. Have few patches on this and pass queue reorg for
next stage1, so will try to push them out.

Honza