Avoid unsatisfied-external-reference errors in static inlines.

Avoid unsatisfied-external-reference errors in static inlines.

Commit 9c727360b neglected the lesson we've learned before:
protect references to backend global variables with #ifndef FRONTEND.

Since there's already a place for static inlines in this file,
move the just-converted functions to that stanza.  Undo the
entirely gratuitous de-macroization of RelationGetNumberOfBlocks
(that one may be okay, since it has no global variable references,
but it's also pointless).

Per buildfarm.