[JSC][ARMv7] Make use of isNullable in OMG

https://bugs.webkit.org/show_bug.cgi?id=298224

Reviewed by Justin Michaud.

Sync WasmOMGIRGenerator32_64.cpp with

https://commits.webkit.org/298875@main.
Canonical link: https://commits.webkit.org/299490@main

Author: aoikonomopoulos