This adds a new ExternalPointer type, which is an Internal type that is
used for ExternalReferences and other pointer values, like the pointers
into the asm.js heap. It also adds a PointerConstant operator, which we
use to represents these raw constants (we can probably remove that
particular operator again once WebAssembly ships with the validator).

R=mvstanton@chromium.org
BUG=v8:5267,v8:5270

Review-Url: https://codereview.chromium.org/2494753003
Cr-Commit-Position: refs/heads/master@{#40923}