[StatepointLowering] Use Constant instead of TargetConstant for undef value (#81635)

Prevents isel errors when trying to lower gc relocate of undef value
(which turns into CopyToReg of TargetConstant). Such relocates may occur
after DCE (e.g. after GVN removes some dead blocks) if there are not
passes like instcombine scheduled after to clean them up.

Fixes #80294

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Co-authored-by: Matt Arsenault <arsenm2@gmail.com>