Clean up Bitmap allocation size computation.

In addition to making this do what is intended in both debug and
release, this removes some of the last uses of sk_64_isS32 and
sk_64_asS32 so that they can be removed.

Test: refactoring CL. Existing unit tests still pass.
Change-Id: I4cd9fb124719f8e7e23db6bf268cbda64c29d283