[PM/Unswitch] Teach the new simple loop unswitch to handle loop

invariant PHI inputs and to rewrite PHI nodes during the actual
unswitching.

The checking is quite easy, but rewriting the PHI nodes is somewhat
surprisingly challenging. This should handle both branches and switches.

I think this is now a full featured trivial unswitcher, and more full
featured than the trivial cases in the old pass while still being (IMO)
somewhat simpler in how it works.

Next up is to verify its correctness in more widespread testing, and
then to add non-trivial unswitching.

Thanks to Davide and Sanjoy for the excellent review. There is one
remaining question that I may address in a follow-up patch (see the
review thread for details) but it isn't related to the functionality
specifically.

Differential Revision: https://reviews.llvm.org/D32699

llvm-svn: 302867

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