Need to allow constant expressions in args of qp that need constants

### Expected Behaviour

The expression
qp(pyear, ell_py, 10, rho_py, sigma_py)
should properly expand if `pyear` and `ell_py` were previously defined as literals or (more generally) constant expressions.

### Actual Behaviour
The expression is not expanded, causing problems in a later stage of the compiler.

Start by reproducing the qp expression with pyear and ell_py defined as literals or constant expressions, then trace the compiler's qp expansion and constant-expression handling. Done means the expression expands correctly before the later compiler stage, without the reported failure.

Written by the indexing model from the issue text.