[VectorCombine] optimize alignment for load transform

Here's another minimal step suggested by D93229 / D93397 .
(I'm trying to be extra careful in these changes because
load transforms are easy to get wrong.)

We can optimistically choose the greater alignment of a
load and its pointer operand. As the test diffs show, this
can improve what would have been unaligned vector loads
into aligned loads.

When we enhance with gep offsets, we will need to adjust
the alignment calculation to include that offset.

Differential Revision: https://reviews.llvm.org/D93406