Always inline SeqNext and SeqRecheck

Always inline SeqNext and SeqRecheck

The intention of the work done in fb9f95502 was that these functions are
inlined.  I noticed my compiler isn't doing this on -O2 (gcc version
15.2.0).  Also, clang version 20.1.8 isn't inlining either.  Fix by
marking both of these functions as pg_attribute_always_inline to avoid
leaving this up to the compiler's heuristics.

A quick test with a Seq Scan on a table with a single int column running
a query that filters all 1 million rows in the WHERE clause yields a
3.9% speedup on my Zen4 machine.

Author: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/CAApHDvrL7Q41B=gv+3wc8+AJGKZugGegUbBo8FPQ+3+NGTPb+w@mail.gmail.com