Fix compressing conditionals

Only transform foo() ? EXP(x) : EXP(y) into EXP(foo() ? x : y) if EXP has no
side effects.

Fix #710

lib/compress.js

@@ -2356,6 +2356,7 @@ merge(Compressor.prototype, {
         if (consequent instanceof AST_Call
             && alternative.TYPE === consequent.TYPE
             && consequent.args.length == alternative.args.length
+            && !consequent.expression.has_side_effects(compressor)
             && consequent.expression.equivalent_to(alternative.expression)) {
             if (consequent.args.length == 0) {
                 return make_node(AST_Seq, self, {