[llvm] Remove uses of Type::getPointerTo() (NFC)

Partial progress towards removing in-tree uses of `getPointerTo()`,
by employing the following options:

* Drop the call entirely if the sole purpose of it is to support a no-op
  bitcast (remove the no-op bitcast as well).

* Replace with `PointerType::get()`/`PointerType::getUnqual()`

This is a NFC cleanup effort.

Reviewed By: barannikov88

Differential Revision: https://reviews.llvm.org/D155232