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[Statepoint] Handle `undef` operands in statepoint.

Authored by dantrushin on Jan 14 2021, 11:35 AM.

Description

[Statepoint] Handle undef operands in statepoint.

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 a use without 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).

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D94703