[BasicAA] Fix AA bug on dynamic allocas and stackrestore

Summary:
BasicAA has special logic for unescaped allocas, which normally applies
equally well to dynamic and static allocas. However, llvm.stackrestore
has the power to end the lifetime of dynamic allocas, without referring
to them directly.

stackrestore is already marked with the most conservative memory
modification attributes, but because the alloca is not escaped, the
normal logic produces incorrect results. I think BasicAA needs a special
case here to teach it about the relationship between dynamic allocas and
stackrestore.

Fixes PR40118

Reviewers: gbiv, efriedma, george.burgess.iv

Subscribers: hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D55969

llvm-svn: 349945