[AArch64] Add a tablegen pattern for SQXTN2.

Converts concat_vectors(Vd, trunc(smin(smax Vm, -2^n), 2^n-1) to
sqxtn2(Vd, Vm). Deliberately not handling v2i64 ~> v2i32 as the
min/max nodes are not legal (same thing we did for the SQXTN
patterns in https://reviews.llvm.org/D103263).

Differential Revision: https://reviews.llvm.org/D116105