Undo CopyForDeref assertion in const qualif

[beepster4096]

Fixes #147733 caused by #145513

This code in fact does not run only on runtime MIR.

[rustbot]

Some changes occurred to constck

cc @fee1-dead

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

[rustbot]

r? @madsmtm

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[madsmtm]

r? saethlin (since you reviewed the original PR)

[RalfJung]

What kind of MIR does const_checks get run on, if it's not runtime MIR?

[beepster4096]

It runs on post-cleanup analysis MIR right here right before the lowering to runtime MIR.