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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102034 91177308-0d34-0410-b5e6-96231b3b80d8