Calculate liveness on the fly for local registers.

Calculate liveness on the fly for local registers.

When working top-down in a basic block, substituting physregs for virtregs, the use-def chains are kept up to date. That means we can recognize a virtreg kill by the use-def chain becoming empty.

This makes the fast allocator independent of incoming kill flags.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103866 91177308-0d34-0410-b5e6-96231b3b80d8