Optimizer: add global value, loop, range, and effect-aware optimization

[chrisbbreuer]

Parent: #459
Depends on: #467, #461

Goal

Implement backend-neutral global optimizations over the optimizer IR: redundant work elimination, loop optimization, range reasoning, and guard/bounds-check reduction.

Scope

• Global value numbering/CSE, copy propagation, sparse conditional constant propagation, dead-code elimination, and effect-aware load/store forwarding.
• Dominator-based loop-invariant code motion, induction-variable recognition, strength reduction, loop rotation/unswitching where profitable, and invariant guard hoisting.
• Integer/range/length analysis for overflow-aware arithmetic, packed arrays, strings, typed arrays, and Wasm memory.
• Redundant shape/prototype/tag/bounds/barrier elimination only when invalidation and memory-order proofs remain valid.
• Block layout, branch probability, register-pressure, and code-size cost models.
• Preserve exact bytecode-step/watchdog accounting even when emitted machine operations are removed or moved.

Effect model requirements

Calls, coercions, getters/setters, proxies, iterators, allocation/OOM, GC safepoints, debugger hooks, atomics, shared-memory accesses, and no-GIL mutations must be explicit barriers or modeled effects. Unknown does not mean pure.

Acceptance

• An IR verifier checks dominance, SSA, effect ordering, recovery states, and map liveness after every pass in test builds.
• Each pass has focused positive/negative tests and seeded forced-off/on semantic differentials.
• Moving GC, exceptions, invalidation races, termination, exact steps, atomics, and host effects remain exact.
• Perf: add causal attribution and exact-parent regression infrastructure #461 reports pass-level compile time, code bytes, eliminated operations/guards, and deopt changes.
• Retained passes improve at least one predeclared Bench: define the representative dependency-free performance matrix #460 row without a noise-qualified regression elsewhere.

Foundation

Optimizer CFG/SSA and reducible regions landed under #431 and #438, including a42aaf26.