Run wasm-opt first, remove sign_ext feature

[ascjones]

parity-wasm currently requires the sign-ext feature in order to load a Wasm module for validation containting sign_ext instructions, otherwise we get the unknown opcode 192 error. This feature can cause issues when combined with other crates which depend on it but don't handle that new instruction e.g. wasm-instrument (without sign-ext enabled).

However, in #1280 we use SignExtLowering to remove those instructions during optimization, to allow for compatibility with versions of pallet_contracts which do not yet support that instruction.

This PR reverses the optimize and post_process steps so that optimization happens first, removing the instruction.

Proof it works, building any contract in debug mode, sign_ext feature removed, first with original order now with new order:

[SkymanOne]

Merging on behalf of @ascjones