Do not remove empty lifetime.start/lifetime.end ranges

Summary:
Asan stack-use-after-scope check should poison alloca even if there is
no access between start and end.

This is possible for code like this:
for (int i = 0; i < 3; i++) {
  int x;
  p = &x;
}

"Loop Invariant Code Motion" will move "p = &x;" out of the loop, making
start/end range empty.

PR27453

Reviewers: eugenis

Differential Revision: https://reviews.llvm.org/D22842

llvm-svn: 277068