Extended Description

Currently codegen represents a vector_shuffle
as a node with three operands. The third operand
is a mask, which must be a BUILD_VECTOR node of
constant integers (a different kind of node that
produces a result of the right vector type isn't
good enough). This causes problems during type
legalization because it is not always possible to
create a legal BUILD_VECTOR usable as the vector
shuffle mask. (LegalizeDAG ignores the legality
or not of the mask argument, but LegalizeTypes
insists on legalizing the types of all nodes,
which I think is the right thing to do).

I think the best plan is to incorporate the mask
directly into the vector shuffle node, by turning
into a node with 2+N operands: the first two the
same as now, followed by the N mask elements.

This seems fairly straightforward to do, but involves
a lot of tedious work. I don't have time for it right
now so I've opened this PR in the hope that someone
else will take it on :)