[wasm] Exploit unallocated zero page to remove null checks in jiterpreter (#86403)

[wasm] Exploit unallocated zero page to remove null checks in jiterpreter (#86403)

* Fuse null and length checks for getelema1 and getchr
* Sense whether zero page optimizations are safe based on the location of the emscripten stack and contents of memory near zero
* Improve jiterpreter stats formatting