Functions calling __builtin_eh_return must have a frame pointer.

The code in X86TargetLowering::LowerEH_RETURN() assumes that a frame
pointer exists, but the frame pointer was forced by the presence of
llvm.eh.unwind.init which isn't guaranteed.

If llvm.eh.unwind.init is actually required in functions calling
eh.return (is it?), we should diagnose that instead of emitting bad
machine code.

This should fix the dragonegg-x86_64-linux-gcc-4.6-test bot.

llvm-svn: 158961