[ARM] GlobalISel: Fix stack-use-after-scope bug.

Summary:
Lifetime extension wasn't triggered on the result of BuildMI because the
reference was non-const. However, instead of adding a const, I've
removed the reference entirely as RVO should kick in anyway.

Reviewers: rovka, bkramer

Reviewed By: bkramer

Subscribers: aemerson, rengolin, dberris, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D29124

llvm-svn: 293059