[PATCH] Add pg_lfind8_nonzero()

Hi hackers,

I'd like to add pg_lfind8_nonzero() to optimize some code like this:

```
	for (i = 0; i < numberOfAttributes; i++)
	{
		if (isnull[i])
		{
			hasnull = true;
			break;
		}
	}
```

With pg_lfind8_nonzero(), we can write the code like this:

```
	if (likely(numberOfAttributes > 0))
		hasnull = pg_lfind8_nonzero((uint8 *) isnull, numberOfAttributes);
```

The pg_lfind8_nonzero() is faster because we can handle 8 bool values at a time.

v1-0001 add the pg_lfind8_nonzero().

v1-0002 use the pg_lfind8_nonzero() in some places.

v1-0003 add a extension "test_patch" only for testing, the test like this:

```
    for (i = 0; i < 1000; i++)
    {
        for (int j = 0; j < natts; j++)
        {
            if (isnull[j])
            {
                hasnull = true;
                break;
            }
        }
    }
=======================
    for (i = 0; i < 1000; i++)
    {
        if (likely(natts > 0))
            hasnull = pg_lfind8_nonzero((uint8 *) isnull, natts);
    }
```

create extension test_patch;
# 4 is the natts
select test_head(4);
select test_patch(4);

Test result:
natts: 4   head: 1984ns  patch: 2094ns
natts: 8   head: 3196ns  patch: 641ns
natts: 16  head: 4589ns  patch: 752ns
natts: 32  head: 8036ns  patch: 1152ns
natts: 64  head: 19367ns patch: 2455ns
natts: 128 head: 33445ns patch: 4018ns

Looking forward to your comments!

--
Regards,
ChangAo Chen