[GlobalISel] IRTranslator: Return the scalar for <1 x Ty> constant vectors

Summary:
<1 x Ty> is not a legal vector type in LLT, we shouldn’t build G_MERGE_VALUES
instruction for them.

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

Reviewed By: qcolombet

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

Differential Revision: https://reviews.llvm.org/D30948

llvm-svn: 297792