Fix logical error when detecting lifetime intrinsics.

Fix logical error when detecting lifetime intrinsics.

Don't replace a gep/bitcast with 'undef' because that will form a "free(undef)"
which in turn means "unreachable". What we wanted was a no-op. Instead, analyze
the whole tree and look for all the instructions we need to delete first, then
delete them second, not relying on the use_list to stay consistent.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@136752 91177308-0d34-0410-b5e6-96231b3b80d8