Make sure total loop body weight is preserved in loop peeling

Summary:
Regardless how the loop body weight is distributed, we should preserve
total loop body weight. i.e. we should have same weight reaching the body of the loop
or its duplicates in peeled and unpeeled case.

Reviewers: mkuper, davidxl, anemet

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28179

llvm-svn: 290833