fix(hll_family): Fixed PFMERGE wrong merge operation

[mkaruza]

Fixed bug in hllMergeDense that writes to wrong register.

Fixes #4750

[Niennienzz]

I don't think that I understand the code fully, haha, I am just reading the PR title... Just want to confirm that the destination key should be treated as a source if it exists. And because HLL should always count unique values, that's why the examples I made in #4750 should not accumulate counts from destination. But it doesn't change the fact that destination is still a source. Here are more examples:

$> PFADD destination 1 2 3
(integer) 1
$> PFMERGE destination     # 'destination' is a source, even with no additional keys to merge.
OK
$> PFCOUNT destination
(integer) 3

$> PFADD key 4 5
(integer) 1
$> PFMERGE destination key # This is merging 'destination' and 'key', and saving the result to 'destination'.
OK
$> PFCOUNT destination
(integer) 5

Please ignore if the behaviour is already correct. I just wanted to confirm.

[mkaruza]

Thank you @Niennienzz i have updated PR so it treats destination as valid source if exits (and fixes merge operation to produce correct values)