Introduce @llvm.experimental.deoptimize

Summary:
This intrinsic, together with deoptimization operand bundles, allow
frontends to express transfer of control and frame-local state from
one (typically more specialized, hence faster) version of a function
into another (typically more generic, hence slower) version.

In languages with a fully integrated managed runtime this intrinsic can
be used to implement "uncommon trap" like functionality.  In unmanaged
languages like C and C++, this intrinsic can be used to represent the
slow paths of specialized functions.

Note: this change does not address how `@llvm.experimental_deoptimize`
is lowered.  That will be done in a later change.

Reviewers: chandlerc, rnk, atrick, reames

Subscribers: llvm-commits, kmod, mjacob, maksfb, mcrosier, JosephTremoulet

Differential Revision: http://reviews.llvm.org/D17732

llvm-svn: 263281