Improve possible performance regression

Improve possible performance regression

Commit ce62f2f2a0a introduced calls to GetIndexAmRoutineByAmId() in
lsyscache.c functions.  This call is a bit more expensive than a
simple syscache lookup.  So rearrange the nesting so that we call that
one last and do the cheaper checks first.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/E1tngY6-0000UL-2n%40gemulon.postgresql.org