Revert 1c021c64 "[SCEV] Model ptrtoint(SCEVUnknown) cast not as unknown, but...

Revert 1c021c64 "[SCEV] Model ptrtoint(SCEVUnknown) cast not as unknown, but as zext/trunc/self of SCEVUnknown"

> While we indeed can't treat them as no-ops, i believe we can/should
> do better than just modelling them as `unknown`. `inttoptr` story
> is complicated, but for `ptrtoint`, it seems straight-forward
> to model it just as a zext-or-trunc of unknown.
>
> This may be important now that we track towards
> making inttoptr/ptrtoint casts not no-op,
> and towards preventing folding them into loads/etc
> (see D88979/D88789/D88788)
>
> Reviewed By: mkazantsev
>
> Differential Revision: https://reviews.llvm.org/D88806

It caused the following assert during Chromium builds:

  llvm/lib/IR/Constants.cpp:1868:
  static llvm::Constant *llvm::ConstantExpr::getTrunc(llvm::Constant *, llvm::Type *, bool):
  Assertion `C->getType()->isIntOrIntVectorTy() && "Trunc operand must be integer"' failed.

See code review for a link to a reproducer.

This reverts commit 1c021c64.