Eliminate return-accumulator temps in EH/lock by sinking returns

[richlander]

Summary

When a method's result is computed inside a try, catch, finally-guarded, or lock body and the ret sits after the region closes, the importer reconstructs a synthetic accumulator local:

try { V = x + 1; } finally { ... } return V;   // reconstructed
try { return x + 1; } finally { ... }          // original source

#611 (the definite-assignment dead-init drop) keeps that temp but declares it bare, which recompiles opcode-exact for the simple shapes its DA walk can prove. It deliberately bails on leave and lock, so the two-return try/finally (TryFinallyTwoReturns) and the lock body (ClassicLock) keep diverging — and even the shapes it fixes still render the unnatural temp form rather than the source.

ReturnSinkingPass rewrites the accumulator back to source form: it sinks each feeding store into a return in place — adjacent StoreLocal V; return V; pairs, and the terminal fall-through stores of a try body, catch arm, lock body, or if/else arm that a trailing return V follows.

Soundness

All-or-nothing by construction: the pass acts only on a local that is never address-taken and whose every load is a Return operand, and applies a candidate only when every store and every load is consumed by the plan. switch is excluded — a switch expression is lowered through its own result accumulator, so per-arm returns would diverge.

Results

• CfgSampleClass compile-back: exact 93 -> 95 (TryFinallyTwoReturns, ClassicLock now exact); all EH/lock return-accumulators render the natural try { return e; }.
• Compile-check arbiter unchanged across 35K+ BCL methods (System.Private.CoreLib / System.Linq / System.Collections) — no new malformed or binding errors (measured with the pass toggled on/off).
• Gate updated: the five EH/lock shapes pinned exact; KnownDiffs trimmed to the residual docket.
• All 433 decompiler tests + 18 analysis tests green.

Relationship to #611

Complementary, not a replacement. #611 is the conservative dead-init drop; this restructures the temp away entirely, producing more natural C# and closing the leave/lock cases #611 leaves behind.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com