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[ThinLTO] Represent relative BF using a scaled representation .

Description

[ThinLTO] Represent relative BF using a scaled representation .

Summary:
The current integer representation of relative block frequency prevents
representing relative block frequencies below 1. This change uses a 8 of
the 29 bits to represent the decimal part by using a fixed scale of -8.

Reviewers: tejohnson, davidxl

Subscribers: mehdi_amini, inglorion, llvm-commits

Differential Revision: https://reviews.llvm.org/D43520