This patch implements tracking of live registers. This is used to look for free
vector registers. It works by walking up the CFG from the reloads all the way
to the spills, accumulating the register units being used.
This implementation caches the live register units used by each MBB for faster
Note: Live register tracking relies on MBB liveins/outs being maintained
correctly, which is implemented in a follow-up patch. So this patch will still
not generate correct code for all but some simple cases.