Add LLVM 'unreachable' instruction to list of allowed instructions.

native_client_sdk/src/doc/reference/pnacl-bitcode-abi.rst

 ==============================
 PNaCl Bitcode Reference Manual
 ==============================
 
 .. contents::
    :local:
    :backlinks: none
    :depth: 3
 
 Introduction
@@ skipping 350 matching lines @@
 * ``bitcast``
 
   The pointer argument of a ``bitcast`` instruction must be a *inherent* pointer
   (see :ref:`pointer types <bitcode_pointertypes>`).
 
 * ``icmp``
 * ``fcmp``
 * ``phi``
 * ``select``
 * ``call``
+* ``unreachable``
 
 .. _bitcode_allocainst:
 
 ``alloca``
 ----------
 
 The only allowed type for ``alloca`` instructions in PNaCl bitcode is i8. The
 size argument must be an i32. For example:
 
 .. naclcode::
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     declare i1 @llvm.nacl.atomic.is.lock.free(i32 <byte_size>, i8* <address>)
 
 The ``llvm.nacl.atomic.is.lock.free`` intrinsic is designed to
 determine at translation time whether atomic operations of a certain
 ``byte_size`` (a compile-time constant), at a particular ``address``,
 are lock-free or not. This reflects the C11 ``atomic_is_lock_free``
 function from header ``<stdatomic.h>`` and the C++11 ``is_lock_free``
 member function in header ``<atomic>``. It can be used through the
 ``__nacl_atomic_is_lock_free`` builtin.