LLVM IR Conversion: support zero-dimensional memrefs

The spec allows zero-dimensional memrefs to exist and treats them essentially
as single-element buffers.  Unlike single-dimensional memrefs of static shape
<1xTy>, zero-dimensional memrefs do not require indices to access the only
element they store.  Add support of zero-dimensional memrefs to the LLVM IR
conversion.  In particular, such memrefs are converted into bare pointers, and
accesses to them are converted to bare loads and stores, without the overhead
of `getelementptr %buffer, 0`.

PiperOrigin-RevId: 240579456