GlobalISel: make sure debugging variables are appropriately elided in release builds.

Summary:
There are two variables here that break. This change constrains both of them to
debug builds (via DEBUG() or #ifndef NDEBUG).

Reviewers: bkramer, t.p.northover

Subscribers: mehdi_amini, vkalintiris

Differential Revision: https://reviews.llvm.org/D26421

llvm-svn: 286300