[llvm][TypeSize] Consider TypeSize of '0' to be fixed/scalable-agnostic. (#72994)

This patch allows adding any quantity to a zero-initialized TypeSize,
such
that e.g.:

  TypeSize::Scalable(0) + TypeSize::Fixed(4) == TypeSize::Fixed(4)
  TypeSize::Fixed(0) + TypeSize::Scalable(4) == TypeSize::Scalable(4)

This makes it easier to implement add-reductions using TypeSize where
the 'scalable' flag is not yet known before starting the reduction.

(this PR follows on from #72979)