Code cleanup for heap_freeze_tuple.

Robert Haas <rhaas@postgresql.org>

Commit: 7386089d23c748af142ec7e3668fa0dd164eaf99
Author: Robert Haas <rhaas@postgresql.org>
Date: 2012-03-26T15:03:06Z
Releases: 9.2.0
Code cleanup for heap_freeze_tuple.

It used to be case that lazy vacuum could call this function with only
a shared lock on the buffer, but neither lazy vacuum nor any other
code path does that any more.  Simplify the code accordingly and clean
up some related, obsolete comments.

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