[Coroutine] Remain alignment information when merging frame variables

Summary: This is to address bug48712.
The solution in this patch is that when we want to merge two variable a
into the storage frame of variable b only if the alignment of a is
multiple of b.
There may be other strategies. But now I think they are hard to handle
and benefit little. Or we can implement them in the future.

Test-plan: check-llvm

Reviewers: jmorse, lxfind, junparser

Differential Revision: https://reviews.llvm.org/D94891