llvm dev - Nov 2020 - Analyze SCEV expression after IR is removed clarification

In short:

Shall analysis of a SCEV expression be allowed even though part of the IR that
was analyzed to build the SCEV expression has been removed?

In more detail with background:

About a month ago I landed https://reviews.llvm.org/D87494 where SCEV is used to
do additional debug-info salvaging in LSR. The idea was to compute (and store) a
SCEV of what llvm.dbg.value intrinsics inside the loop refers to and then after
LSR transformation compare these SCEVs with the current SCEVs of PHI-nodes of
the loop header. If any of these match (with a possible offset) then the
llvm.dbg.value intrinsic can be updated to refer to the PHI-node (with a
possible offset in the DIExpression).

This use-case obviously depends on being able to analyze a SCEV where parts of
the corresponding IR has been removed.

Now last week https://bugs.llvm.org/show_bug.cgi?id=48166 was reported which
raises the question if this is allowed use-case or not.

Clearly if the Value that a SCEVUnknown refers to has been deleted then that
renders the entire SCEV useless. But if only IR that SCEV has successfully
"analyzed through" is removed then I would argue that the SCEV
expression should still be analyzable.

Thoughts, comments and clarifications are welcome.

-Markus
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