Collapse dual-constant guard-returns to the bare condition, not c && true

[richlander]

What

FoldGuardReturn folds if (c) return A; return B; into a short-circuit boolean when either arm is a bool constant. When both arms are constants it still ran the generic formula, appending the then literal as a dead term:

• BothPositive (if (a > 0) { if (b > 0) return true; } return false;) folded to return a > 0 && b > 0 && true;
• the dual if (c) return false; return true; to return !c || false;

How

When the two arms are opposite bool constants, the condition itself is the result — if (c) return true; return false; is return c;, and the dual is return !c;. Special-case that ahead of the generic fold and reuse the (already detached) condition directly: no extra &&/|| term, and the condition is still evaluated, so the rewrite is sound even when it has side effects.

Equal-constant arms (the dead if (c) return true; return true;) fall through to the generic path; they do not arise in compiled code.

Soundness / scope

This is an output-fidelity fix. BothPositive already diverges from its fixture's nested-if-to-literal IL — an intentional, idiomatic boolean-fold over-render (the same policy that renders IsNullOrEmpty as return value is null || value.Length == 0;). It stays on the compile-back docket as before, but now renders the idiomatic return a > 0 && b > 0; instead of the noisy && true.

Numbers

• Compile-back fixture counts unchanged: exact 157, docket 8, recompile-fail 537 (System.Linq unchanged).
• Decompiler tests 440 to 441 (new BothPositive fold test).
• Main suite 1140 green.

Found via the #604 --compile-back oracle while reviewing the ternary/branch-sense docket.