LLVM IR Dialect: unify call and call0 operations

When the LLVM IR dialect was implemented, TableGen operation definition scheme
did not support operations with variadic results.  Therefore, the `call`
instruction was split into `call` and `call0` for the single- and zero-result
calls (LLVM does not support multi-result operations).  Unify `call` and
`call0` using the recently added TableGen support for operations with Variadic
results.  Explicitly verify that the new operation has 0 or 1 results.  As a
side effect, this change enables clean-ups in the conversion to the LLVM IR
dialect that no longer needs to rely on wrapped LLVM IR void types when
constructing zero-result calls.

PiperOrigin-RevId: 236119197