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ARROW-7098: [Java] Improve the performance of comparing two memory blocks

We often use the 8-4-1 paradigm to compare two blocks of memory:
1. First compare by 8-byte blocks in a loop
2. Then compare by 4-byte blocks in a loop
3. Last compare by 1-byte blocks in a loop

It can be proved that the second loop runs at most once. So we can replace the loop with a if statement, which will save us a comparison and two jump operations.

According to the discussion in https://github.com/apache/arrow/pull/5508#discussion_r343973982, loops can be expensive.

Closes #5790 from liyafan82/fly_1108_if and squashes the following commits:

d747762ae <liyafan82>  Improve the performance of comparing two memory blocks

Authored-by: liyafan82 <fan_li_ya@foxmail.com>
Signed-off-by: Micah Kornfield <emkornfield@gmail.com>