This is a fix for PR30318.
Clang may generate IR where an alloca is already live when entering a
BB with lifetime.start. In this case, conservatively extend the
alloca lifetime all the way back to the block entry.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281784 91177308-0d34-0410-b5e6-96231b3b80d8