[VP] Propagate align parameter attr on VP load/store to ISel

This patch fixes a case where the 'align' parameter attribute on the
pointer operands to llvm.vp.load and llvm.vp.store was being dropped
during the conversion to the SelectionDAG. The default alignment
equal to the ABI type alignment of the vector type was kept. It also
updates the documentation to reflect the fact that the parameter
attribute is now properly supported.

Reviewed By: simoll

Differential Revision: https://reviews.llvm.org/D114422