cgirgen: fix translation of defs

[zerbina]

Summary

Properly translate local definitions from the MIR to the CgNode IR.
While the previous implementation didn't produce the wrong results (so
far), it was based on incorrect assumptions.

Details

• add tbBody and use it for translating the body of control-flow
  constructs (mnkIf, mnkBlock, etc.)
• replace the inArgBlock counter with the inUnscoped flag
• remove the tbStmt overload that's now only used in a single place
• instead of storing cnkDef nodes in the defs list, store the
  information required to produce the nodes; this is more efficient, as
  it means less work during clean-up (the contents of the defs seq
  don't require destruction)

Translation of defs

A def for a local/global in the MIR means "locations starts to exist
and is accessible from all connected basic blocks within the same scope
(mnkScope) to which the only control-flow path also visits the def".
The CgNode IR doesn't have an explicit notion of "scope" as the MIR
does, and placing a cnkDef in some control-flow- related construct
(e.g., cnkIf) can produce the wrong code when the construct doesn't
open a new scope itself (i.e., its body is not wrapped in an
mnkScope).

Previously, translating a mnkDef to an assignment, with the cnkDef
placed at the start of the enclosing scope (this is also referred to as
lifting the def), was performed for all defs within an argument block.
However, argument blocks are wholly unrelated to the problem described
above, and the associated comment was also wrong: a cnkStmtList
doesn't open a scope.

Due to a lot of code being located in some argument block, this
incorrect implementation still produced the correct results so far, but
changes to how MIR code is generated or how some of the MIR passes
operate could break this at any time.

The correct thing to look for is whether a definition is inside a
control-flow constructs (if, block, etc.). If it is, and there's no
mnkScope in-between, the definition needs to be lifted into the
closest surrounding scope.

An alternative approach would be introducing a cnkScope node for the
CgNode IR and then leaving the work to the code generators. The
reason
for not going that route is that keeping the lifting logic in cgirgen
for now makes it easier to change later on.

Future work

• only locals that are used outside of their enclosing control-flow
  construct need to be lifted; an analysis pass could detect whether
  this is the case
• now that the semantics are clearer, the lifting of defs in
  the destructor injection pass can be removed

[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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