[SelectionDAG] Don't promote the alignment of allocas beyond the stack alignment.

[SelectionDAG] Don't promote the alignment of allocas beyond the stack alignment.

allocas in LLVM IR have a specified alignment. When that alignment is
specified, the alloca has at least that alignment at runtime.

If the specified type of the alloca has a higher preferred alignment,
SelectionDAG currently ignores that specified alignment, and increases
the alignment. It does this even if it would trigger stack realignment.
I don't think this makes sense, so this patch changes that.

I was looking into this for SVE in particular: for SVE, overaligning
vscale'ed types is extra expensive because it requires realigning the
stack multiple times, or using dynamic allocation. (This currently isn't
implemented.)

I updated the expected assembly for a couple tests; in particular, for
arg-copy-elide.ll, the optimization in question does not increase the
alignment the way SelectionDAG normally would. For the rest, I just
increased the specified alignment on the allocas to match what
SelectionDAG was inferring.

Differential Revision: https://reviews.llvm.org/D79532