implement raw pointer comparisons in librustc

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.
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.