Changed sign of LastCallToStaticBouns

Summary:
I think it is much better this way.
When I firstly saw line:
  Cost += InlineConstants::LastCallToStaticBonus;
I though that this is a bug, because everywhere where the cost is being reduced
it is usuing -=.

Reviewers: eraman, tejohnson, mehdi_amini

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D23222

llvm-svn: 278290