Bug 1787794 - Part 1: Prefer BytecodeEmitter::emitUnpickN() over JSOp::Unpick. r=arai

This shaves off one byte in byte code size:

`dis(function(){ [...a] = [] })`

Before:
00092:  27  True                        # [] [][Symbol.iterator]().next [][Symbol.iterator]() [] true
00093:  27  Unpick 1                    # [] [][Symbol.iterator]().next [][Symbol.iterator]() true []
00095:  27  TryDestructuring            # [] [][Symbol.iterator]().next [][Symbol.iterator]() true []

After:
00092:   1  True                        # [] [][Symbol.iterator]().next [][Symbol.iterator]() [] true
00093:   1  Swap                        # [] [][Symbol.iterator]().next [][Symbol.iterator]() true []
00094:   1  TryDestructuring            # [] [][Symbol.iterator]().next [][Symbol.iterator]() true []

Differential Revision: https://phabricator.services.mozilla.com/D155850