Take index type into account in dynamic memory explicit bounds checks

In [the general case](https://github.com/bytecodealliance/wasmtime/blob/6c5184809db3a92de4ee0c718c403bedc9a9ff4f/cranelift/wasm/src/code_translator/bounds_checks.rs#L258-L262), we do a bunch of `uadd_overflow_trap`s, but we shouldn't need to worry about overflow for 32-bit Wasm memories on 64-bit targets.