[AArch64][CodeGen] Remove redundant vector negations before concat

This adds a new canonicalization rule to replace concats of truncated
negations with a negation of the concatenated truncates, e.g.

    (concat_vectors (v4i16 (truncate (not (v4i32)))),
                    (v4i16 (truncate (not (v4i32)))))
   ->
    (not (concat_vectors (v4i16 (truncate (v4i32))),
                         (v4i16 (truncate (v4i32)))))

Doing this allows avoiding redundant negations being emitted in
certain cases.

Reviewed By: peterwaller-arm

Differential Revision: https://reviews.llvm.org/D137433