Add ptrmask intrinsic

This patch adds a ptrmask intrinsic which allows masking out bits of a
pointer that must be zero when accessing it, because of ABI alignment
requirements or a restriction of the meaningful bits of a pointer
through the data layout.

This avoids doing a ptrtoint/inttoptr round trip in some cases (e.g. tagged
pointers) and allows us to not lose information about the underlying
object.

Reviewers: nlopes, efriedma, hfinkel, sanjoy, jdoerfert, aqjune

Reviewed by: sanjoy, jdoerfert

Differential Revision: https://reviews.llvm.org/D59065

llvm-svn: 368986