Enabling IPCP by default

[Jan Hubicka <jh at suse dot cz>]

Hi,
Since most of issues with IPCP should be fixed now and it should be as
strong as possible with the elementary textbook quality algorithm it
uses, I would like to enable it by default.  I've tested it on SPEC and
C++ behcmarks yeterday and didn't measured any significant improvments.
There is quite a lot of clonning happening now (as can be seen on size
increase) on SPECint, but the benchmark performance don't care much.
Today I fixed some isses and added code to avoid code size growth, so
I expect IPCP to be mostly neutral.  Will re-run the tests tonight.

To some degree I would say it is expected as all those codebases are quite well
hand optimized and this is kind of optimization one does by hand if needed.

Ipcp can now run in tow modes: in pure constant propagation when clonning is
not happening (well, in fact it does. We clone the function and the remove the
original as in place replacement is not implement (yet)) and with clonning.
With clonning overall unit growth is limited to 10% and ipcp performs very
simplistic analysis of effectivity and will clone functions until the limit is
met in priority order.

On CC1 binarry I get following results:
			stripped size	clonned functions
no ipcp			8863256		0
ipcp only		8773432		45
ipcp&clonning		8772344		154
(with unlimited clonning we get about 180 clones)

Additionally ipcp and ipcp&clonning binary seems consistently 0.7%-1.5% faster
on compiling C objects.

Since IPCP seems essentially free for compile time (i.e. intraprocedural
analysis is performed for inlining anyway and interprocedural step is very
cheap when nothing is transformed) and cause 1% code size savings on cc1 
and speeds it up enough to pay back, I would like to propose:
  - enable IPCP for -O2 and -Os
  - enable IPCP clonning for -O3

Ipcp also carries basic IPA infrastructure we want to keep excercised
(jump function analysis, clonning, propagation to function bodies and
infrastructure to solve cgraph updates for future whopr mode).

I would still like to work on better ipcp cost model (i.e. estimates on how
much function will simplify with constant propagation) and also to allow
producing multiple clones when function is called with multiple constant
arguments.

If there are no complains, I will enable ipcp as proposed after remaining
patches are tested and comitted (that would be about day after tomorrow)
Honza