Make LiveVariables an instance variable of PHIElimination.
Remove ancient references to 'atomic' phis in PHIElimination that don't really
make sense anymore.
Fix a typo.
Add support for updating LiveIntervals to PHIElimination. If LiveIntervals are
present, it currently verifies them with the MachineVerifier, and this passed
all of the test cases in 'make check' (when accounting for existing verifier
errors). There were some assertion failures in the two-address pass, but they
also happened on code without phis and look like they are caused by different
kill flags from LiveIntervals.

The only part that doesn't work is the critical edge splitting heuristic,
because there isn't currently an efficient way to update LiveIntervals after
splitting an edge. I'll probably start by implementing the slow fallback and
test that it works before tackling the fast path for single-block ranges. The
existing code that updates LiveVariables is fairly slow as it is.

There isn't a command-line option for enabling this; instead, just edit
PHIElimination.cpp to require LiveIntervals.