GlobalISel: Translate ConstantAggregateZero vectors

Reviewers: qcolombet, aditya_nandakumar, dsanders, ab, t.p.northover, javed.absar

Reviewed By: qcolombet

Subscribers: dberris, rovka, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D30259

llvm-svn: 297509