[SROA] Reuse existing lifetime markers if possible

Summary:
If the underlying alloca did not change, we do not necessarily need new
lifetime markers. This patch adds a check and reuses the old ones if
possible.

Reviewers: reames, ssarda, t.p.northover, hfinkel

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68900

llvm-svn: 374692