FlattenSpec: flattening a module with no instaces should be a no-op (#1868)

* FlattenSpec: flattening a module with no instaces should be a no-op

* Fix problem when flattening/inlining a lone module

Fix an edge case bug in InlineInstances where a circuit containing a
lone module is flattened/inlined. This now properly special cases the
situation of an empty indexMap which before had to be of length >= 1.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

* Simplify rename logic in InlineInstances

Co-authored-by: Jack Koenig <koenig@sifive.com>
Co-authored-by: Albert Magyar <albert.magyar@gmail.com>
Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

* Mea culpa

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

Co-authored-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>
Co-authored-by: Jack Koenig <koenig@sifive.com>
Co-authored-by: Albert Magyar <albert.magyar@gmail.com>