Teach fast isel calls and rets about stdcall.

stdcall is callee-pop like thiscall, so the thiscall changes already did most
of the work for this.  This change only opts stdcall in and adds tests.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@275414 91177308-0d34-0410-b5e6-96231b3b80d8