Rename GistTranslateStratnum() to GistTranslateCompareType()

Rename GistTranslateStratnum() to GistTranslateCompareType()

Follow up to commit 630f9a43cec.  The previous name had become
confusing, because it doesn't actually translate a strategy number but
a CompareType into a strategy number.  We might add the inverse at
some point, which would then probably be called something like
GistTranslateStratnum.

Reviewed-by: Mark Dilger 
Discussion: https://www.postgresql.org/message-id/flat/E72EAA49-354D-4C2E-8EB9-255197F55330@enterprisedb.com