[GlobalOpt] Simplify __cxa_atexit elimination

[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).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353603 91177308-0d34-0410-b5e6-96231b3b80d8