Add 'cold' attribute to functions

[Diego Novillo]

This patch adds a new function attribute 'cold' to functions.

Other than recognizing the attribute, the patch does little else.
It changes the branch probability analyzer so that edges into
blocks postdominated by a cold function are given low weight.

I have two follow up patches in progress.  One to add the 'cold'
attribute to Clang and another one to propagate the cold
attribute on the dominator tree (edges into blocks dominated by a
cold call would be given low weight).

OK to commit?  Chandler, I will need help committing the patch as
I don't have write access to the repo.


Thanks.  Diego.
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