Revert r342457 "Fixes removal of dead elements from PressureDiff (PR37252)."

This broke the lit tests on a bunch of buildbots, e.g.
http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-ubuntu-fast/builds/36679

> Reviewed By: MatzeB
>
> Differential Revision: https://reviews.llvm.org/D51495

llvm-svn: 342482