Allow non-btree speculative insertion indexes

Allow non-btree speculative insertion indexes

Previously, only btrees were supported as the arbiter index for
speculative insertion because there was no way to get the equality
strategy number for other index methods.  We have this now (commit
c09e5a6a016), so we can support this.

At the moment, only btree supports unique indexes, so this does not
change anything in practice, but it would allow another index method
that has amcanunique to be supported.

Co-authored-by: Mark Dilger <mark.dilger@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/E72EAA49-354D-4C2E-8EB9-255197F55330@enterprisedb.com