Update lang ref.

The loop identifier metadata can be used to specify additional

per-loop metadata. Any operands after the first operand can be treated

as user-defined metadata. For example the ``llvm.loop.unroll.count``

suggests an unroll factor to the loop unroller:

as user-defined metadata. For example the ``llvm.loop.interleave.count``

suggests an interleave factor to the loop interleaver:

.. code-block:: llvm

      br i1 %exitcond, label %._crit_edge, label %.lr.ph, !llvm.loop !0

    ...

    !0 = metadata !{ metadata !0, metadata !1 }

    !1 = metadata !{ metadata !"llvm.loop.unroll.count", i32 4 }

    !1 = metadata !{ metadata !"llvm.loop.interleave.count", i32 4 }

'``llvm.loop.vectorize``' and '``llvm.loop.interleave``'

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

0 or if the loop does not have this metadata the width will be

determined automatically.

'``llvm.loop.unroll``'

^^^^^^^^^^^^^^^^^^^^^^

Metadata prefixed with ``llvm.loop.unroll`` are loop unrolling

optimization hints such as the unroll factor. ``llvm.loop.unroll``

metadata should be used in conjunction with ``llvm.loop`` loop

identification metadata. The ``llvm.loop.unroll`` metadata are only

optimization hints and the unrolling will only be performed if the

optimizer believes it is safe to do so.

'``llvm.loop.unroll.enable``' Metadata

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This metadata either disables loop unrolling or suggests that the loop

be unrolled fully. The first operand is the string

``llvm.loop.unroll.enable`` and the second operand is a bit.  If the

bit operand value is 0 loop unrolling is disabled. A value of 1

indicates that the loop should be fully unrolled. For example:

.. code-block:: llvm

   !0 = metadata !{ metadata !"llvm.loop.unroll.enable", i1 0 }

   !1 = metadata !{ metadata !"llvm.loop.unroll.enable", i1 1 }

'``llvm.loop.unroll.count``' Metadata

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This metadata suggests an unroll factor to the loop unroller. The

first operand is the string ``llvm.loop.unroll.count`` and the second

operand is a positive integer specifying the unroll factor. For

example:

.. code-block:: llvm

   !0 = metadata !{ metadata !"llvm.loop.unroll.count", i32 4 }

If the trip count of the loop is less than the unroll count the loop

will be partially unrolled.

If a loop has both a ``llvm.loop.unroll.enable`` metadata and

``llvm.loop.unroll.count`` metadata the behavior depends upon the

value of the ``llvm.loop.unroll.enable`` operand.  If the value is 0,

the loop will not be unrolled.  If the value is 1, the loop will be

unrolled with a factor determined by the ``llvm.loop.unroll.count``

operand effectively ignoring the ``llvm.loop.unroll.enable`` metadata.

'``llvm.mem``'

^^^^^^^^^^^^^^^