Corruptly merge constants with explicit and implicit alignments.

Constant merge can merge a constant with implicit alignment with one that has
explicit alignment. Before this change it was assuming that the explicit
alignment was higher than the implicit one, causing the result to be under
aligned in some cases.

Fixes pr17815.

Patch by Chris Smowton!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194506 91177308-0d34-0410-b5e6-96231b3b80d8