[WebAssembly] wasm64: fix memory.init operand types

[WebAssembly] wasm64: fix memory.init operand types

I had assumed they would all become in i64, but this is not necessary as long as data segments stay 32-bit, see:
https://github.com/WebAssembly/memory64/blob/master/proposals/memory64/Overview.md

Differential Revision: https://reviews.llvm.org/D85552