Change the looping stack detection code

In some special case (e.g. signal handlers, hand written assembly) it is
valid to have 2 stack frame with the same CFA value. This CL change the
looping stack detection code to report a loop only if at least 3
consecutive frames have the same CFA.

Differential revision: http://reviews.llvm.org/D12699

llvm-svn: 247133