[Statepoint] Enable cross block relocates w/vreg lowering

This change is mechanical, it just removes the restriction and updates tests.  The key building blocks were submitted in 31342eb6 and 8fe2abc1.

Note that this (and preceeding changes) entirely subsumes D83965.  I did includes a couple of it's tests.

From the codegen changes, an interesting observation: this doesn't actual reduce spilling, it just let's the register allocator do it's job.  That results in a slightly different overall result which has both pros and cons over the eager spill lowering.  (i.e. We'll have some perf tuning to do once this is stable.)