[MemorySSA] Be less aggressive with @llvm.lifetime.start

This is a workaround to keep LLVM from incorrectly removing stores in rare
cases. It has not yet been committed, and -- due to the direction we seem to
want to take MSSA in -- probably never will be.

It should be sufficient to work around the problem until we agree on a good way
forward.

Additional context: https://reviews.llvm.org/D43269

Change-Id: I53cace1c1e4f617335bef299e9e13d15f37d42c4