Extend the tight-loop lane to while and do-while statements

[lahma]

Summary

for-loops with structurally-Normal bodies (expression statements, var/let/const, if/else chains — nothing that routes a break/continue/return/label) bypass per-iteration completion bookkeeping, suspension probes and per-statement ceremony via JintForStatement.TightForBody. while and do-while loops with the identical body shapes still paid all of it — a Completion struct and value-liveness check per iteration, two suspendable probes, and full _body.Execute dispatch.

This extends the same lane to while and do-while: the shared IsTightBodyShape predicate decides eligibility, and eligible bodies run through the same ExecuteDiscarded path — test, discarded body statements and a deferred-error poll per iteration, with amortized constraints (timeout/cancellation/memory) driven through the existing shared countdown. Bodies with lexical declarations stay on the generic path (while/do-while have no loop-env flattening machinery, so a let still needs its per-iteration block environment), as do observable-completion contexts (eval/global completion values), debug mode, exact constraints and suspendable (generator/async) frames.

Benchmarks (default jobs, adjacent A/B on latest main)

New FunctionLocalNumberLoopBenchmark rows (the while/do-while twins of the existing for-loop accumulators; committed alongside):

Benchmark	main	PR	Δ
WhileAccumulator	5.807 ms	2.496 ms	−57.0%
DoWhileCounter	6.298 ms	2.299 ms	−63.5%

Allocations unchanged (904 B both sides — the lane removes per-iteration Completion structs, which don't allocate).

SunSpider rows with while loops (crypto-md5/sha1, string-unpack-code, bitops-nsieve-bits, string-tagcloud) are neutral — their hot loops are for-loops or top-level while loops whose completion values are observable, so they don't hit the new lane; this PR targets function-local while/do-while, which the suite otherwise didn't cover.

Semantics (12-case REPL verification, cold + cache-armed)

while/do-while sums, do-while runs body once when test false, break/continue/labeled-continue bodies stay generic and correct, let in body forces the generic path with correct closures, observable completion values (eval) force generic path, empty body with side-effectful test, exceptions propagate from tight bodies with correct loop state.

Gates

• Jint.Tests: 3,597 (net10.0) + 3,534 (net472) passed, 0 failed
• Jint.Tests.PublicInterface: 114 × 2 TFMs passed
• Jint.Tests.CommonScripts: 28 × 2 TFMs passed
• Test262: 99,429 passed, 0 failed

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