Use the same XS body for B::IV::RV and B::PV::RV.

Use the same XS body for B::IV::RV and B::PV::RV.

This reduces the object code size by about 600 bytes on this platform.

The input typemap checking makes no distinction between different SV types,
so the XS body will generate the same C code, despite the different XS
"types". So there is no change in behaviour from doing newXS like this,
compared with the old approach of having a (near) duplicate XS body.
We should fix the typemap checking.