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ARROW-7378: [C++][Gandiva] Fix loop vectorization in gandiva

- for slice offsets, use local variables instead of memory references.
  The memory references make it harder for the loop vectorization to happen.
- added a test to verify loop vectorization is happening.

Closes #6019 from pravindra/arrow-7378 and squashes the following commits:

4368a210c <Pindikura Ravindra> fix lint error
f5d7cf04b <Pindikura Ravindra> ARROW-7378:  Fix loop vectorization in gandiva

Authored-by: Pindikura Ravindra <ravindra@dremio.com>
Signed-off-by: Pindikura Ravindra <ravindra@dremio.com>