[GlobalOpt] Simplify __cxa_atexit elimination

cxxDtorIsEmpty checks callers recursively to determine if the
__cxa_atexit-registered function is empty, and eliminates the
__cxa_atexit call accordingly.

This recursive check is unnecessary as redundant instructions and
function calls can be removed by early-cse and inliner. In addition,
cxxDtorIsEmpty does not mark visited function and it may visit a
function exponential times (multiplication principle).

llvm-svn: 353603