[HotColdSplitting] Allow outlining single-block cold regions

It can be profitable to outline single-block cold regions because they
may be large.

Allow outlining single-block regions if they have over some threshold of
non-debug, non-terminator instructions. I chose 3 as the threshold after
experimenting with several internal frameworks.

In practice, reducing the threshold further did not give much
improvement, whereas increasing it resulted in substantial regressions.

Differential Revision: https://reviews.llvm.org/D53824

llvm-svn: 345524

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