fix terminal expression used as construction operand

[zerbina]

Summary

Fix using a terminal expressions as an object construction operand
resulting in incorrect run-time behaviour.

Details

Elimination of unreachable expression didn't handle object constructors
properly, not adding the terminal expression to the resulting statement
list. For example:

  Obj(a: x, b: (stmt; return; 1))
  # was turned into just
  discard x
  # `stmt; return` was erroneously omitted 

Since the surrounding statement was marked as not returning, the lack
of terminator statement caused incorrect MIR to be produced by
mirgen.

The terminal expression is now added to the produced statement list,
and a test is added to make sure eliminating construction expressions
works.

[saem]

/merge

[github-actions]

Merge requested by: @saem

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
• I haven't investigated it, but the incorrect MIR might have also led to compiler crashes, depending on which MIR passes were run