ARM: Avoid dereferencing end() in ARMFrameLowering::emitEpilogue

This fixes the crash from PR29072, where the MachineBasicBlock::iterator
wasn't being properly checked against MachineBasicBlock::end() before
iterating.  This was another bug exposed by the new
ilist::iterator::operator*() assertion from r279314.

This testcase is poor quality.  bugpoint couldn't reduce any further,
and I haven't had time to dig into what's going on so I can't invent a
better one.  I didn't even get good CHECK lines in: this is just a
crasher.

I'm committing anyway since this is a real crash with an obvious fix,
but I'll leave PR29072 open and ask an ARM maintainer to help improve
the testcase.

llvm-svn: 279391