Give a small negative bias to giant edge bundles.

This helps compile time when the greedy register allocator splits live
ranges in giant functions. Without the bias, we would try to grow
regions through the giant edge bundles, usually to find out that the
region became too big and expensive.

If a live range has many uses in blocks near the giant bundle, the small
negative bias doesn't make a big difference, and we still consider
regions including the giant edge bundle.

Giant edge bundles are usually connected to landing pads or indirect
branches.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157174 91177308-0d34-0410-b5e6-96231b3b80d8

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