When the inliner merges allocas, it must keep the larger alignment

For safety, the inliner cannot decrease the allignment on an alloca when
merging it with another.

I've included two variants of the test case for this: one with DataLayout
available, and one without. When DataLayout is not available, if only one of
the allocas uses the default alignment (getAlignment() == 0), then they cannot
be safely merged.