wasm: refactor extended instructions

The extended instructions starting with opcode `0xFC` are refactored
to make the work the same as the SIMD instructions. This means a
`Mir` instruction no longer contains a field 'secondary'. Instead,
we use the `payload` field to store the index into the extra list
which contains the extended opcode value. In case of instructions
such as 'memory.fill' which also have an immediate value, such
values will also be stored in the extra list right after the
instruction itself. This makes each `Mir` instruction smaller.

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