[PATCH] Avoid unnecessary code execution in Instrument.c when TIMING is FALSE

Hi,

Even when using EXPLAIN ANALYZE with TIMING=FALSE, the functions
InstrStopNode(), InstrEndLoop(), and InstrAggNode() in Instrument.c
still execute code related to the "starttime", "counter", "firsttuple",
"startup", and "total" fields within the Instrumentation structure.
These operations are unnecessary when timing is disabled, and since
these functions are called very frequently, I have created a patch to
address this.

As far as I can tell, this change has no side effects and clarifies the
intent of each line, but please let me know if you notice any issues.

Best regards,
H.S.