[ScheduleDAGInstrs] Re-factor for based on review feedback. NFC.

Summary:
Re-factor some code to improve clarity and style based on review
comments from http://reviews.llvm.org/D18093.

Reviewers: MatzeB, mcrosier

Subscribers: MatzeB, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D19128

llvm-svn: 266372