[Statepoint] Handle 'undef' operand tied to def

[skatkov]

Summary

FixupStatepoints pass does not take into account the undef use
it skips may have a tied def. So when defs are handled pass
considers that tied-use should be spilled and triggers an assert.

FixupStatepoints should skip undef def as well.

[dantrushin]

LGTM.

BTW, we can completely delete tied dead,undef pair. But additional complexity might not worth it.