Fix typo in DeadCodeElimination.cpp
Fix typo in DeadCodeElimination.cpp

<code>prescence</code> -> <code>presence</code>
<code>structrually</code> -> <code>structurally</code>
khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift