[Statepoint] Handle 'undef' operand tied to def

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.

Reviewers: reames, dantrushin
Reviewed By: dantrushin
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D95858