Improve heapgetpage() performance, overhead from serializable

Hi,

Several loops which are important for query performance, like heapgetpage()'s
loop over all tuples, have to call functions like
HeapCheckForSerializableConflictOut() and PredicateLockTID() in every
iteration.

When serializable is not in use, all those functions do is to to return. But
being situated in a different translation unit, the compiler can't inline
(without LTO at least) the check whether serializability is needed. It's not
just the function call overhead that's noticable, it's also that registers
have to be spilled to the stack / reloaded from memory etc.

On a freshly loaded pgbench scale 100, with turbo mode disabled, postgres
pinned to one core. Parallel workers disabled to reduce noise. All times are
the average of 15 executions with pgbench, in a newly started, but prewarmed
postgres.

SELECT * FROM pgbench_accounts OFFSET 10000000;
HEAD:
397.977

removing the HeapCheckForSerializableConflictOut() from heapgetpage()
(incorrect!), to establish the baseline of what serializable costs:
336.695

pulling out CheckForSerializableConflictOutNeeded() from
HeapCheckForSerializableConflictOut() in heapgetpage(), and avoiding calling
HeapCheckForSerializableConflictOut() in the loop:
339.742

moving the loop into a static inline function, marked as pg_always_inline,
called with static arguments for always_visible, check_serializable:
326.546

marking the always_visible, !check_serializable case likely():
322.249

removing TestForOldSnapshot() calls, which we pretty much already decided on:
312.987

FWIW, there's more we can do, with some hacky changes I got the time down to
273.261, but the tradeoffs start to be a bit more complicated. And 397->320ms
for something as core as this, is imo worth considering on its own.

Now, this just affects the sequential scan case. heap_hot_search_buffer()
shares many of the same pathologies. I find it a bit harder to improve,
because the compiler's code generation seems to switch between good / bad with
changes that seems unrelated...

I wonder why we haven't used PageIsAllVisible() in heap_hot_search_buffer() so
far?

Greetings,

Andres Freund