fix(GeneralizeProofs): unreachable! bug

[digama0]

As reported on Zulip.

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[github-actions[bot]]

PR summary 4b793a02b2

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## summary with just the declaration names:
./scripts/no_lost_declarations.sh short <optional_commit>

## more verbose report:
./scripts/no_lost_declarations.sh <optional_commit>

[jcommelin]

It would be good to have a test demonstrating the fix.

[kmill]

I think this triggers the same unreachable bug:

example (l : List Nat) (h : 1 < l.length) : let i := 2; l.get ⟨1, id h⟩ = 0 := by
  generalize_proofs

[mathlib4-merge-conflict-bot]

This pull request has conflicts, please merge master and resolve them.