Reduce more the number of calls to GetMaxBackends()

Reduce more the number of calls to GetMaxBackends()

Some of the code paths changed by aa64f23 can reduce the number of times
GetMaxBackends() is called.  The performance gain is marginal, but most
of the code changed by this commit already did that.  Hence, let's be
clean and apply the same rule everywhere, for consistency.

Some of the code paths, like in deadlock.c, involve only assertions.
These are left unchanged.

Reviewed-by: Nathan Bossart, Robert Haas
Discussion: https://postgr.es/m/YgMpGZhPOjNfS7er@paquier.xyz