Fully raise Roslyn ForEachLoopOperation.ChildOperationsCount #9969

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Goal

Fully raise Roslyn's
Microsoft.CodeAnalysis.Operations.ForEachLoopOperation.ChildOperationsCount
to its authored four-term count expression, without synthetic stack-slot or
struct-receiver temporaries.

This is a focused successor to #9946 and part of the printer-thinning program
tracked by #9371. #9946 should remain bounded to preserving the slot's correct
System.Int32 identity.

Source oracle

The Microsoft.CodeAnalysis.Common 5.0.0 Portable PDB maps the property to
Roslyn commit 6c4a46a31302167b425d5e0a31ea83c9a9aa1d09; dotnet-inspect verifies the
fetched bytes against the PDB checksum.

=>
    (LoopControlVariable is null ? 0 : 1) +
    (Collection is null ? 0 : 1) +
    NextVariables.Length +
    (Body is null ? 0 : 1);

https://github.com/dotnet/roslyn/blob/6c4a46a31302167b425d5e0a31ea83c9a9aa1d09/src/Compilers/Core/Portable/Generated/Operations.Generated.cs#L4213-L4217

Current output after #9946

int S_256 = (LoopControlVariable is not null ? 1 : 0)
    + (Collection is not null ? 1 : 0);
ImmutableArray<IOperation> V_0 = NextVariables;
return S_256 + V_0.Length
    + (base.Body is not null ? 1 : 0);

#9946 removes the incorrect Boolean split and preserves the actual integer
count. Two source-shape gaps remain:

1. the now-correct, single-use arithmetic stack value S_256 is not inlined
   into the return expression;
2. the ImmutableArray<IOperation> receiver temporary V_0 is not
   reconstructed as NextVariables.Length.

Proof obligations

• Preserve the authored evaluation order and one evaluation of each property.
• Prove that removing S_256 cannot duplicate, reorder, or narrow the
  multi-term integer computation.
• Prove that reconstructing NextVariables.Length preserves the value-type
  receiver and defensive-copy behavior represented by the IL.
• Keep the rule narrow and typed; do not add printer guesses or broad
  single-use-local elimination.
• Identify the normative pass owner and pipeline position. In particular,
  determine whether identity-preserving passes must be followed by another
  bounded inlining opportunity rather than rerunning a broad early pass.

Acceptance criteria

• The pinned Roslyn 5.0.0 property decompiles as one direct four-term return
  expression with no S_256 or V_0 declaration. Equivalent inverted null
  ternaries are acceptable.
• Scenario-adjacent tests separately gate arithmetic-stack inlining and the
  value-type property-receiver reconstruction.
• Tests demonstrate decline when inlining could duplicate evaluation, change
  order, or alter struct receiver behavior.
• Focused Release tests and the fixed 14-assembly corpus show no unrelated
  residual, fidelity, or validity regression.
• The owning design records the bounded rule and explicit exclusions.

The remaining Boolean-to-ArgumentRefKind split in
TypeRef.FunctionPointerParameterText is a separate enum-identity problem and
is outside this issue.