Added a temporary option to avoid critical edges splitting.

This teaches the register coalescer to be less prone to split critical
edges. I am currently benchmarking this with the new (post-coalescer)
scheduler. I plan to enable this by default and remove the option as
soon as misched is enabled.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167758 91177308-0d34-0410-b5e6-96231b3b80d8