Document a bug in loop-convert and fix one of its subcases.

Summary: Now that we prioritize copying trivial types over using const-references where possible, I found some cases where, after the transformation, the loop was using the address of the local copy instead of the original object.

Reviewers: klimek

Subscribers: alexfh, cfe-commits

Differential Revision: http://reviews.llvm.org/D13431

llvm-svn: 249300