hio: Relax rules for calling GetVisibilityMapPins()
GetVisibilityMapPins() insisted on the buffer1/buffer2 being in a specific
order. This required checks at the callsite. As a subsequent patch will add
another callsite, move related logic into GetVisibilityMapPins().
Discussion: https://postgr.es/m/
20230403190030.fk2frxv6faklrseb@awork3.anarazel.de