The substitution was missing a `\@@_combo_sc_shape:n`
This would mean that a `bx/scit` would be substituted to `b/it` rather
than to `b/scit`.

If a slanted font is not provided, fontspec setup automatic sl -> it
substitutions in \@@_declare_shape_slanted:nn. But if a small caps
font was provided, an scit shape is defined but there is no
corresponding scsl -> scit substitution.

Fix this by adapting the code of \@@_declare_shapes_smcaps:nn into
\@@_declare_shape_slanted:nn.

Use a similar fix in \@@_declare_shapes_bx:nn for the bx/scsl -> bx/scit
substitution.