polish: skip creating extra reference for safely re-used node

[JLHwung]

Q	A
Fixed Issues?	Fixes #10717
Patch: Bug Fix?	Yes
Tests Added + Pass?	Yes
License	MIT

In this PR we skip creating extra reference when the left of ?? is a static reference, which is safely to reused when later unfolding into conditional comparison.

Compared to the master branch, this PR generate less code for the common case var bar = foo ?? "bar"
Before:

var bar = (_foo = foo) !== null && _foo !== void 0 ? _foo : "bar"
// minified (52 B)
// var bar=null!==(_foo=foo)&&void 0!==_foo?_foo:"bar";

After:

var bar = foo !== null && foo !== void 0 ? foo : "bar"
// minified (43 B)
// var bar=null!==foo&&void 0!==foo?foo:"bar";

[nicolo-ribaudo]

What about using isConstantExpression, or maybe better maybeGenerateMemoised?

[JLHwung]

Good idea! I end up with maybeGenerateMemoised because it is rare that the left is a literal --even if it is, creating a reference other than inlining this literal should costs less bytes in most situations, given that we generate [_]+ as identifier name for literals.

[JLHwung]

if ref is non-null, it is always cloned from node.left.

[nicolo-ribaudo]

We need to figure out why CircleCI isn't running for your PRs