[InlineFunction] Only replace call if there are VarArgs to forward.

Summary:
There is no need to replace the original call instruction if no
 VarArgs need to be forwarded. 

Reviewers: davide, rnk, majnemer, efriedma

Reviewed By: efriedma

Subscribers: eraman, llvm-commits

Differential Revision: https://reviews.llvm.org/D40412

llvm-svn: 319947