[SimplifyCFG] Skip sinking common lifetime markers of `alloca`.

Summary:
- Similar to the workaround in fix of PR30188, skip sinking common
  lifetime markers of `alloca`. They are mostly left there after
  inlining functions in branches.

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66950

llvm-svn: 370376