Fix double shufti's vector end false positive (#325)

Fix double shufti's vector end false positive (#325)

* Add regression test for double shufti

It tests for false positive at vector edges.

Signed-off-by: Yoan Picchi <yoan.picchi@arm.com>
* Fix double shufti reporting false positives

Double shufti used to offset one vector, resulting in losing one character
at the end of every vector. This was replaced by a magic value indicating a
match. This meant that if the first char of a pattern fell on the last char of
a vector, double shufti would assume the second character is present and
report a match.
This patch fixes it by keeping the previous vector and feeding its data to the
new one when we shift it, preventing any loss of data.

Signed-off-by: Yoan Picchi <yoan.picchi@arm.com>
* vshl() will call the correct implementation

* implement missing vshr_512_imm(), simplifies caller x86 code

* Fix x86 case, use alignr instead

* it's the reverse, the avx512 alignr is incorrect, need to fix

* Make shufti's OR reduce size agnostic

Signed-off-by: Yoan Picchi <yoan.picchi@arm.com>
* Fix test's array size

Signed-off-by: Yoan Picchi <yoan.picchi@arm.com>
* Fix AVX2/AVX512 alignr implementations and unit tests

* Fix Power VSX alignr

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Signed-off-by: Yoan Picchi <yoan.picchi@arm.com>
Co-authored-by: Konstantinos Margaritis <konstantinos@vectorcamp.gr>