compile conditional assigns to assign with ternary operand

[thejh]

This commit implements my suggestion from #193 and does even more. Some compilation examples:
(first line: original, second line: old compilation, third line: new compilation)

if(a)b=foo()
a&&(b=foo())
b=a?foo():b

if(a)b=foo();else b=bar()
a?b=foo():b=bar()
b=a?foo():bar()

if(a)b+=3
a&&(b+=3)
b+=a?3:0

if(a)b.c+=3
a&&(b.c+=3)
-

if(a)b.c+=3;else b.c+=4
a?b.c+=3:b.c+=4
b.c+=a?3:4

It reduces the size of minified jquery by 0,12% (116 bytes). However, it increases
the gzipped size of minified jquery by 26 bytes :( .

[RGustBardon]

if (0 === Math.random()) {
  _ = 0;
}

var _ = '';

if (0 === Math.random()) {
  _ += 42;
}

if (-1 !== _.indexOf('0')) {
  throw new Error();
}

!function() {
  Object.defineProperty(this, '_', {
    get: function() {
      throw new Error();
    }
  });
  if (0 === Math.random()) {
    _ += 42;
  }
}();

!function() {
  Object.defineProperty(this, '_', {
    set: function(v) {
      throw new Error();
    }
  });
  if (0 === Math.random()) {
    _ += 42;
  }
}();

with ({
  get _() {
    throw new Error();
  }
}) {
  if (0 === Math.random()) {
    _ += 42;
  }
}

with ({
  set _(v) {
    throw new Error();
  }
}) {
  if (0 === Math.random()) {
    _ += 42;
  }
}

var Foo = function() {
};

Foo.prototype.toString = function() {
  throw new Error();
};

var _ = new Foo();

if (0 === Math.random()) {
  _ += 42;
}

var Foo = function() {
};

Foo.prototype.valueOf = function() {
  throw new Error();
};

var _ = new Foo();

if (0 === Math.random()) {
  _ += 42;
}