Fix a quadratic algorithm in MachineBranchProbabilityInfo.

The getSumForBlock function was quadratic in the number of successors
because getSuccWeight would perform a linear search for an already known
iterator.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161460 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed