[LangRef] insertelement/extractelement return poison for out of range.

[efriedma]

Summary

We need to clarify the language here. I think poison makes more sense than undef, since it's an undefined operation rather than uninitialized memory. I don't think anything depends on the difference at the moment, though.

[nlopes]

LGTM

llvm/trunk/docs/LangRef.rst

 the position from which to extract the element. The index may be a
 variable of any integer type.
 
 Semantics:
 """"""""""
 
 The result is a scalar of the same type as the element type of ``val``.
 Its value is the value at position ``idx`` of ``val``. If ``idx``
-exceeds the length of ``val``, the results are undefined.
+exceeds the length of ``val``, the result is a
+:ref:`poison value <poisonvalues>`.
 
 Example:
 """"""""
 
 .. code-block:: text
 
       <result> = extractelement <4 x i32> %vec, i32 0    ; yields i32
 

 is an index indicating the position at which to insert the value. The
 index may be a variable of any integer type.
 
 Semantics:
 """"""""""
 
 The result is a vector of the same type as ``val``. Its element values
 are those of ``val`` except at position ``idx``, where it gets the value
-``elt``. If ``idx`` exceeds the length of ``val``, the results are
-undefined.
+``elt``. If ``idx`` exceeds the length of ``val``, the result
+is a :ref:`poison value <poisonvalues>`.
 
 Example:
 """"""""
 
 .. code-block:: text
 
       <result> = insertelement <4 x i32> %vec, i32 1, i32 0    ; yields <4 x i32>