Avoid re-checking for visibility map extension too frequently.

Avoid re-checking for visibility map extension too frequently.

When testing bits (but not when setting or clearing them), we now
won't check whether the map has been extended.  This significantly
improves performance in the case where the visibility map doesn't
exist yet, by avoiding an extra system call per tuple.  To make
sure backends notice eventually, send an smgr inval on VM extension.

Dean Rasheed, with minor modifications by me.