[ARM][ISel] Improve the lowering of vector loads.

When vectors are built from a single value, the ARM lowering issues a
scalar_to_vector node.
This node is then always morphed into a move from the general purpose unit to
the vector unit.
When the value comes from a load, this can be simplified into a vector load to
the right lane.

This patch changes the lowering of insert_vector_elt to expose a vector
friendly pattern in this situation.

This is a step toward fixing <rdar://problem/14170854>.