Do not set branch weight if the branch weight annotation is present.

Summary: ThinLTO will annotate the CFG twice. If the branch weight is set by the first annotation, we should not set the branch weight again in the second annotation because the first annotation is more accurate as there is less optimization that could affect debug info accuracy.

Reviewers: tejohnson, davidxl

Reviewed By: tejohnson

Subscribers: mehdi_amini, aprantl, llvm-commits

Differential Revision: https://reviews.llvm.org/D31228

llvm-svn: 298602