[GlobalISel] IRTranslator: Translate ConstantStruct

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

Reviewed By: qcolombet

Subscribers: rovka, kristof.beyls, javed.absar, igorb, llvm-commits

Differential Revision: https://reviews.llvm.org/D33317

llvm-svn: 303412