This patch adds support for the vector-predicated VP_STORE and
VP_LOAD nodes. We do this in the same way we lower MSTORE and
MLOAD: to regular load/store instructions via intrinsics.
One necessary change was made to SelectionDAGLegalize so that
VP_STORE nodes' operation actions are taken from the stored "value"
operands, in the same vein as STORE or MSTORE.