[DebugInfo] Refine the condition to detect CIEs.

The condition was not accurate enough and could interpret some FDEs in
.eh_frame or 64-bit DWARF .debug_frame sections as CIEs. Even though
such FDEs are unlikely in a normal situation, the wrong interpretation
could hide an issue in a buggy generator.

Differential Revision: https://reviews.llvm.org/D73886