Implement variadic-def statepoint lowering.
Assign N base/derived non-constant pointers to VRegs, pass others
on stack slots, as in old implementation.
So new statepoint looks like this:
reloc1,reloc2,... = STATEPOINT ..., base1, derived1<tied-def0>, base2, derived2<tied-def1>, ...
N is limited by the maximal number of tied registers machine
instruction can have (15 at the moment).
If any of gc.relocates are not in the same basic block (as the case for
invoke statepoint), use old spilling scheme too.
Full change set is available at D81603.