Lift a spilled base/this constructor argument into the chain initializer

[richlander]

What

A base(...)/this(...) argument that carries control flow — the ubiquitous base(message ?? "default") / base(code > 0 ? "x" : null) exception shape — gets spilled to a temporary. The printer's chain lift only fires when the call is the body's first statement, so the spilled-argument call stayed an invalid base(temp); body statement (CS0175) that dropped its argument on recompile (ldarg ldarg call → ldarg call).

The receiver of a constructor's own base/this call is the object under construction — an immutable reference with no observable evaluation effect — so moving an argument's computation past it never reorders anything. The general inliner declines this (it can't yet prove the this receiver isn't a mutable byref struct), exactly as ConstructorChainPass already noted for the receiver itself.

How

• ConstructorChainArgumentPass (new, runs after structuring folds the ??/?:): inlines each single-use argument spill into the chain call, so the call lands as the body's first statement and the existing lift renders : base(args) / : this(args).
• Harness (CompileBack): the oracle ignored the lifted initializer and emitted class skeletons with no base type, so lifted chains couldn't recompile. It now renders the initializer and emits a : Base clause for non-generic in-assembly class bases.

Soundness

• The pass reuses single-store / single-load / address-not-taken safety and only inlines temps in the run immediately preceding the chain call, where the sole prior evaluation is the effect-free under-construction this.
• The base clause is conservative (TypeDefinition, non-generic, non-object only); generic / out-of-assembly bases stay unspelled, costing at most a base-call diff, never a miscompile. On a broad System.Linq corpus the base clause left recompile-fail unchanged (424 → 424).

Numbers

Compile-back fixture corpus:

metric	before	after
exact opcode match	147	151 (+4)
opcode diff (docket)	14	13
recompile fail	528	525 (no new failures)

CtorChainSamples base(message), base(message ?? "default"), and this(value.ToString()) are now opcode-exact; the remaining ctor#2 diff is the orthogonal ternary-sense equivalence. Decompiler tests 432 pass; main suite 1140 pass (9 skipped).

Found via the --compile-back oracle (#604).