dialects(wasm): implement sign extension operators

[mooori]

What should be done?

These ops interpret lower bits of the operand as the value of a smaller type and sign extend it. Motivation:

• Their semantics might not be captured correctly in the arith dialect (bug: incorrect lowering of wasm's i32.extend8_s, i32.extend16_s #960)
• Handling them in the wasm dialect allows generating more efficient code

Ops

• i32.extend8_s: extend a signed 8-bit integer to a 32-bit integer feat: add wasm dialect #986
• i32.extend16_s: extend a signed 16-bit integer to a 32-bit integer feat: lower i32.extend16_s through wasm dialect #998
• i64.extend8_s: extend a signed 8-bit integer to a 64-bit integer feat: lower i64.extend*_s through wasm dialect #1008
• i64.extend16_s: extend a signed 16-bit integer to a 64-bit integer feat: lower i64.extend*_s through wasm dialect #1008
• i64.extend32_s: extend a signed 32-bit integer to a 64-bit integer feat: lower i64.extend*_s through wasm dialect #1008