Use pg_nextpower2_* in a few more places

Back in f0705bb62, we added pg_nextpower2_32 and pg_nextpower2_64 to
efficiently obtain the next power of 2 of a given number using an
intrinsic function to find the left-most 1 bit.

In d025cf88b and 02a2e8b44, I added some usages of these new functions
but I didn't quite get all of them done.   The attached replaces all
of the remaining ones that I'm happy enough to go near.

The ones that I left behind are ones in the form of:

while (reqsize >= buflen)
{
   buflen *= 2;
   buf = repalloc(buf, buflen);
}

The reason I left those behind is that I was too scared that I might
introduce an opportunity to wrap buflen back around to zero again.  At
the moment the repalloc() would prevent that as we'd go above
MaxAllocSize before we wrapped buflen back to zero again.  All the
other places I touched does not change the risk of that happening.

It would be nice to get rid of doing that repalloc() in a loop, but it
would need a bit more study to ensure we couldn't wrap or we'd need to
add some error checking code that raised an ERROR if it did wrap.  I
don't want to touch those as part of this effort.

I've also fixed up a few places that were just doubling the size of a
buffer but used a "while" loop to do this when a simple "if" would
have done.  Using an "if" is ever so slightly more optimal since the
condition will be checked once rather than twice when the buffer needs
to increase in size.

I'd like to fix these for PG15.

David