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