ping^2 [rfc][patch] Document RETURN_EXPR better

http://gcc.gnu.org/ml/gcc-patches/2009-12/msg00690.html [cc'ing gcc since it might be the better forum for this]

Re: Unnecessary PRE optimization

> In general it will be tricky for latter passes to clean up the messes. > The fundamental problem is that the address computation is exposed to > PRE prematurely (for a given target  ) at GIMPLE level. Yeah, i'm not sure PRE can really do anything different here. I also think you would have a ve