Operand types for the 0xFD vector opcode space. Add a vector instruction's types here, alongside its decoding.

Four tables, one per immediate shape, matching wasm_decode_simd:

• type_of/1 for the plain lane-wise instructions, which is nearly all of them and where the type is a function of the opcode alone;
• lane_op/1 for extract_lane and replace_lane, which also need the lane count so the immediate can be bounds-checked;
• mem_op/1 for the vector loads and stores, whose natural alignment is the width of the access rather than 16: v128.load32_splat reads four bytes;
• lane_mem_op/1 for the lane-indexed loads and stores, which need both.

Kept apart from wasm_validate_code for the same reason the decoder is: this opcode space is larger than every other prefix combined, and inlining it would bury the rules that are actually interesting.