A new pass to provide specific optimizations for certain well-known library

A new pass to provide specific optimizations for certain well-known library
calls. The pass visits all external functions in the module and determines
if such function calls can be optimized. The optimizations are specific to
the library calls involved. This initial version only optimizes calls to
exit(3) when they occur in main(): it changes them to ret instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@21522 91177308-0d34-0410-b5e6-96231b3b80d8