Run LiveVariables instead of computing liveness locally in -regalloc=fast.

This actually makes everything slower, but the plan is to have isel add <kill>
flags the way it is already adding <dead> flags. Then LiveVariables can be
removed again.

When ignoring the time spent in LiveVariables, -regalloc=fast is now twice as
fast as -regalloc=local.

llvm-svn: 102034