[lld] Migrate away from PointerUnion::dyn_cast (NFC) (#228356)

Note that PointerUnion::dyn_cast has been soft deprecated in
PointerUnion.h:

  // FIXME: Replace the uses of is(), get() and dyn_cast() with
  //        isa<T>, cast<T> and the llvm::dyn_cast<T>

Literal migration would result in dyn_cast_if_present (see the
definition of PointerUnion::dyn_cast), but this patch uses dyn_cast
where the operands are known to be nonnull.

The operands are known to be nonnull in one of two ways:

- Preceding checks: In BPSectionOrderer.cpp,
  reloc.getReferentInputSection() is called a few lines above (and the
  caller checks r.referent.isNull()), guaranteeing the PointerUnion is
  nonnull.

- Downstream code: The else or fallthrough branch unconditionally
  calls cast<...> or dyn_cast<...> on the pointer, or dereferences it.

Assisted-by: Antigravity
GitOrigin-RevId: 5a04c0a62947b12d1392abc71a5c72e88874e830