[RDF] Switch RefMap in liveness calculation to use lane masks

[RDF] Switch RefMap in liveness calculation to use lane masks

This required reengineering of some of the part of liveness calculation,
including fixing some issues caused by the limitations of the previous
approach. The current code is not necessarily the fastest, but it should
be functionally correct (at least more so than before). The compile-time
performance will be addressed in the future.

llvm-svn: 284609