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.

llvm-svn: 275414
diff --git a/llvm/lib/Target/X86/X86FastISel.cpp b/llvm/lib/Target/X86/X86FastISel.cpp
index 182731f..fae4527 100644
--- a/llvm/lib/Target/X86/X86FastISel.cpp
+++ b/llvm/lib/Target/X86/X86FastISel.cpp
@@ -1151,6 +1151,7 @@
   if (CC != CallingConv::C &&
       CC != CallingConv::Fast &&
       CC != CallingConv::X86_FastCall &&
+      CC != CallingConv::X86_StdCall &&
       CC != CallingConv::X86_ThisCall &&
       CC != CallingConv::X86_64_SysV)
     return false;
@@ -3025,6 +3026,7 @@
   case CallingConv::WebKit_JS:
   case CallingConv::Swift:
   case CallingConv::X86_FastCall:
+  case CallingConv::X86_StdCall:
   case CallingConv::X86_ThisCall:
   case CallingConv::X86_64_Win64:
   case CallingConv::X86_64_SysV: