[LangRef] get.active.lane.mask can produce poison

[SjoerdMeijer]

Summary

We had already specified that second argument n of this intrinsic is n > 0, but now add to this that the result is a poison value if this is not the case.

[efriedma]

LGTM

llvm/docs/LangRef.rst

 ::
 
       %m[i] = icmp ult (%base + i), %n
 
 where ``%m`` is a vector (mask) of active/inactive lanes with its elements
 indexed by ``i``,  and ``%base``, ``%n`` are the two arguments to
 ``llvm.get.active.lane.mask.*``, ``%icmp`` is an integer compare and ``ult``
 the unsigned less-than comparison operator.  Overflow cannot occur in
-``(%base + i)`` and its comparison against ``%n`` with ``%n > 0``, as it is
-performed in integer numbers and not in machine numbers. The above is
-equivalent to:
+``(%base + i)`` and its comparison against ``%n`` as it is performed in integer
+numbers and not in machine numbers.  If ``%n`` is ``0``, then the result a
+poison value. The above is equivalent to:
 
 ::
 
       %m = @llvm.get.active.lane.mask(%base, %n)
 
 This can, for example, be emitted by the loop vectorizer in which case
 ``%base`` is the first element of the vector induction variable (VIV) and
 ``%n`` is the loop tripcount. Thus, these intrinsics perform an element-wise