Fix a collision with exported variables with short names

[jridgewell]

In some cases, it's possible to generate a collision between an exported variable declaration (which cannot be mangled) and a local reference. Eg,

// input
const n = null;
export const o = {o: n};

// options:
{ module: true, reduce_vars: false }

// output
const o=null;
export const o={o:o};

[fabiosantoscode]

This is looking great to merge but there's a TODO comment so I don't know

[jridgewell]

Removed. My intention was to check that #233 wasn't going to affect this, too.

[fabiosantoscode]

Got it 👍 thanks!