[IR] Allow the 'align' param attr on vectors of pointers

This patch extends the available uses of the 'align' parameter attribute
to include vectors of pointers. The attribute specifies pointer
alignment element-wise.

This change was previously requested and discussed in D87304.

The vector predication (VP) intrinsics intend to use this for scatter
and gather operations, as they lack the explicit alignment parameter
that the masked versions use.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D115161