Don't require LiveVariables for PHIElimination. Enable critical edge splitting

when LiveVariables is available.

The -split-phi-edges is now gone, and so is the hack to disable it when using
the local register allocator. The PHIElimination pass no longer has
LiveVariables as a prerequisite - that is what broke the local allocator.
Instead we do critical edge splitting when possible - that is when
LiveVariables is available.

llvm-svn: 89213