[LangRef] update text for shufflevector

[spatel]

Summary

D72467 updated the shufflevector instruction to include a constant mask rather than a mask operand. The LangRef text was vague enough to still make sense, but it would be better to update here too, so there's no confusion about valid mask values. The text here is mostly copied from the documentation code comments for "class ShuffleVectorInst".

[efriedma]

Mentioning "UndefMaskElem" probably doesn't make sense. Maybe should be "undef"?

[spatel]

Patch updated:
Rearranged text a bit more and removed reference to the 'UndefMaskElem' code constant.

[efriedma]

This version lost the "constant integer or undef values" bit?

[spatel]

Patch updated:
Restore description of constant mask elements as integer or undef. That seemed redundant to me before, but it's better to state that explicitly, so we don't let weird stuff like constant expressions into the picture.

[efriedma]

LGTM

llvm/docs/LangRef.rst

 The '``shufflevector``' instruction constructs a permutation of elements
 from two input vectors, returning a vector with the same element type as
 the input and length that is the same as the shuffle mask.
 
 Arguments:
 """"""""""
 
 The first two operands of a '``shufflevector``' instruction are vectors
-with the same type. The third argument is a shuffle mask whose element
-type is always 'i32'. The result of the instruction is a vector whose
-length is the same as the shuffle mask and whose element type is the
+with the same type. The third argument is a shuffle mask vector constant
+whose element type is ``i32``. The mask vector elements must be constant
+integers or ``undef`` values. The result of the instruction is a vector
+whose length is the same as the shuffle mask and whose element type is the
 same as the element type of the first two operands.
 
-The shuffle mask operand is required to be a constant vector with either
-constant integer or undef values.
-
 Semantics:
 """"""""""
 
 The elements of the two input vectors are numbered from left to right
-across both of the vectors. The shuffle mask operand specifies, for each
-element of the result vector, which element of the two input vectors the
-result element gets.
-
-If the shuffle mask is undef, the result vector is undef. If any element
-of the mask operand is undef, that element of the result is undef. If the
-shuffle mask selects an undef element from one of the input vectors, the
-resulting element is undef. An undef mask element prevents a poisoned
-vector element from propagating.
+across both of the vectors. For each element of the result vector, the
+shuffle mask selects an element from one of the input vectors to copy
+to the result. Non-negative elements in the mask represent an index
+into the concatenated pair of input vectors.
+
+If the shuffle mask is undefined, the result vector is undefined. If
+the shuffle mask selects an undefined element from one of the input
+vectors, the resulting element is undefined. An undefined element
+in the mask vector specifies that the resulting element is undefined.
+An undefined element in the mask vector prevents a poisoned vector
+element from propagating.
 
 For scalable vectors, the only valid mask values at present are
 ``zeroinitializer`` and ``undef``, since we cannot write all indices as
 literals for a vector with a length unknown at compile time.
 
 Example:
 """"""""