[AArch64][GlobalISel]: Fix a crash in GlobalIsel in dealing with 16bit uadd.with.overflow.

Summary: AArch64 doesn't support uadd.with.overflow.i16 natively. This change adds a legalization rule to convert the 32bit add result to 16bit. This should fix PR43981.

Reviewers: arsenm, qcolombet, paquette, aemerson

Reviewed By: paquette

Subscribers: wdng, rovka, kristof.beyls, hiraditya, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71587