[Transforms] Add lit test for instcombine on load into vector of overaligned elements.

The result is currently broken in two ways:

 - Valid loads are replaced by poison
 - An array-like layout with padding bytes is assumed

This commit serves as precommit for a patch that addresses the first issue.
The second issue will remain a TODO.

Contributors:
    Sebastian Neubauer <sebastian.neubauer@amd.com>