Cranelift: leak in remove_constant_phis

The `remove_constants_phis` replaces the arugments of an instruction without freeing the original list.

https://github.com/bytecodealliance/wasmtime/blob/2cfa024855f4b542999c7bf547838e2f85b244a5/cranelift/codegen/src/remove_constant_phis.rs#L415

One option for fixing this would be to build up the list to replace it as a vector, and then clearing and re-initializing the old arguments list. However, the `remove_constant_phis` pass might not be necessary at all after @jameysharp's refactoring of the SSA builder, so it would be good to investigate whether it's applying at all to start with.