liftdestructors: generate proper isNil check

[zerbina]

Summary

Fix an internal issue with type-bound operator lifting for closure
types.

Details

Instead of a proper isNil check, generation of the "not nil" check
for
closure environments used the pointer expression itself as the
condition expression of an if.

Since the expression was translated as is and a pointer value is valid
in an if condition position in C, this worked, but it's not correct
according to NimSkull's type system. A proper "not nil" check is
generated now.

[zerbina]

/merge

[github-actions]

Merge requested by: @zerbina

Contents after the first section break of the PR description has been removed and preserved below:

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Notes for Reviewers

• split out from implement a new CGIR and C code generator #1625

[saem]

/merge