if..else dead_code elimination - preserve vars

[boopathi]

if ( 0 ) {
  var x = foo();
}

// or
if (0) var x = bar();

out:

var x;

This can either be done by hoisting vars for all functions before IfStatement is reached, or by analysing the pathToBeRemoved for var declarations.

I tried the latter approach here - https://github.com/boopathi/babel-minify/blob/0fbb45d18b13df53dd225dbd36ef0af88c7190ea/packages/babel-plugin-transform-conditionals/src/replace-with.js#L65-L88

[amasad]

I'd be worried about the effect of hoisting on gzip. If you can prototype it and run the benchmarks it should give you a good idea over the impact. I think your approach in your fork looks good.