Revert "Don't count inreg params when mangling fastcall functions"

This reverts commit r214981.

I'm not sure what I was thinking when I wrote this. Testing with MSVC
shows that this function is mangled to '@f@8':
  int __fastcall f(int a, int b);

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220492 91177308-0d34-0410-b5e6-96231b3b80d8

lib/IR/Mangler.cpp

@@ -72,9 +72,6 @@ static void AddFastCallStdCallSuffix(raw_ostream &OS, const Function *F,
   unsigned ArgWords = 0;
   for (Function::const_arg_iterator AI = F->arg_begin(), AE = F->arg_end();
        AI != AE; ++AI) {
-    // Skip arguments in registers to handle typical fastcall lowering.
-    if (F->getAttributes().hasAttribute(AI->getArgNo() + 1, Attribute::InReg))
-      continue;
     Type *Ty = AI->getType();
     // 'Dereference' type in case of byval or inalloca parameter attribute.
     if (AI->hasByValOrInAllocaAttr())