[RemoveDIs][DebugInfo] Handle RAUW from dead constants (#80837)

Ensure that when a constant value dies, all ValueAsMetadata uses of that
constant in a DebugValueUser will be replaced with a PoisonValue instead
of a nullptr. This has little visible effect currently, as any nullptr metadata
uses in a DebugValueUser would be converted to an empty MDNode in
the end anyway, but this patch adds an assert that would be triggered by
two existing tests without the functional component of this patch:

  llvm/test/Transforms/Inline/delete-function-with-metadata-use.ll
  llvm/test/DebugInfo/X86/array2.ll