[BPI] Add multiplication by scalar operators to BranchProbability

This patch just adds two operators to BranchProbability class:
(BP * scalar) and (BP *= scalar).

Reviewers: junbuml, chandlerc, sanjoy, vsk

Reviewed By: chandlerc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32334

llvm-svn: 300945