New size optimization

[Joern Rennecke]

> I was talking to Anthony Green about this, and he mentioned that it might be
> useful to add a flag to disable loop-invariant-code-motion and then have -Os
> set that flag.  That is, we perhaps should make -Os work like -O2.  -O2 just
> sets a bunch of flags, and the user can override the choices if they are wrong
> for a particular application.  We should perhaps do the same for -Os, making
> it set a bunch of flags that the user can override if the defaults weren't the
> best possible choice.  There is a tradeoff with having too many -f options
> that the user gets confused though.
> 
> Is there any reason why you just disabled the last part of the LICM code, and
> not all of it?  Maybe strength reduction needs some of the info computed by
> LICM.  Or maybe you didn't try disabling the rest of it?  It is probably useful
> to document why this choice was made in a comment.

I didn't try disabling the rest.
The patch I made is simple and allows quick testing.
I don't know if disabling the rest will require any special attention on
data that is used by other code; at any rate, it would require a lot of
indentation changes, or lots of small changes involving the flag to test,
or both.
So before we do that, I think we should decide if we want a new -f option
for this choice - and if yes, how its flag variable is called.