GlobalOpt: Apply fastcc to internal x86_thiscallcc functions

We should apply fastcc whenever profitable.  We can expand this list,
but there are lots of conventions with performance implications that we
don't want to change.

Differential Revision: http://llvm-reviews.chandlerc.com/D2705

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202293 91177308-0d34-0410-b5e6-96231b3b80d8