[Codegen] Change getSpillSize/getReloadSize to LocationSize. NFC (#82636)

This is a small part of #70452, attempting to take a small simpler part
of it in isolation to simplify what remains. It changes the getSpillSize,
getFoldedSpillSize, getRestoreSize and getFoldedRestoreSize methods to return
optional<LocationSize> instead of unsigned. The code is intended to be the
same, keeping the optional<> to specify when there was no size found, with some
minor adjustments to make sure that unknown (~UINT64_C(0)) sizes are handled
sensibly.  Hopefully as more unsigned's are converted to LocationSize's the use
of ~UINT64_C(0) can be cleaned up too.