Handle empty functions with debug info in load/store opt pass

Summary:
In fuctions that contained debug info but were empty otherwise,
the ARM load/store optimizer could abort. This was because
function MergeReturnIntoLDM handled the special case where a
Machine Basic BLock is empty by calling MBB.empty(). However, this
returns false in presence of debug info, although the function
should be considered empty in the eyes of the load/store optimizer.
This has been fixed by handling the case where searching through the
block finds only debug instructions.

Reviewers: rengolin, dexonsmith, llvm-commits, jmolloy

Subscribers: t.p.northover, aemerson, rengolin, samparker

Differential Revision: https://reviews.llvm.org/D23847

llvm-svn: 279820