[CallSiteSplitting] Remove some indirection (NFC).

Summary:
With this patch I tried to reduce the complexity of the code sightly, by
removing some indirection. Please let me know what you think.

Reviewers: junbuml, mcrosier, davidxl

Reviewed By: junbuml

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D40037

llvm-svn: 318593