Experiment with annotating args as "noalias" if we can prove it

[pcwalton]

LLVM arguments accept "noalias", which is nice. We can do type-based alias analysis to set it.

[bstrie]

Nominating for maturity 5.

[thestinger]

We now tag &mut parameters as noalias, I'm going to close this bug and open more specific issues for other potential uses.