Tune basic block annotation algorithm.

Summary:
Instead of using maximum IR weight as the basic block weight, this patch uses the voting algorithm to find the most likely weight for the basic block. This can effectively avoid the cases when some IRs are annotated incorrectly due to code motion of the profiled binary.

This patch also updates propagate.ll unittest to include discriminator in the input file so that it is testing something meaningful.

Reviewers: davidxl, dnovillo

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D19301

llvm-svn: 267519