Currently when spilling statepoint register operands in FixupStatepoints
we do not pay attention that it might be undef. We just generate a
spill, which may lead to verifier error because we have use without a def.
To handle it, let FixupStateponts ignore undef register operands
completely and change them to some constant value when generating
stack map. Use same value as used by ISel for this purpose (0xFEFEFEFE).