When attempting to allocate a blocked register, in the absence of
aliasing, it was possible to assume that a register that was
blocked - by either belonging to an active fixed register, or to
an active unspillable range - could not have possibly be allocated
to another active range (because there'd be an interference otherwise).

With aliasing, that changes. The range we're trying to allocate
may be a double, while the 2 or more active ranges in the paragraph
above may be singles aliasing to the same double slot.

Opportunistically refactored for readability an optimization, and
added some comments.

BUG=681529

Review-Url: https://codereview.chromium.org/2632373004
Cr-Commit-Position: refs/heads/master@{#42474}