auto merge of #9136 : thestinger/rust/ptr, r=alexcrichton

This is mostly for consistency, as you can now compare raw pointers in
constant expressions or without the standard library.

It also reduces the number of `ptrtoint` instructions in the IR, making
tracking down culprits of what's usually an anti-pattern easier.

diff --cc src/librustc/middle/ty.rs
Simple merge