warn & drop #__PURE__ iff IIFE is dropped

[alexlamsl]

@kzc so I think this is the proper fix for #1509 (comment), i.e. without dropping the warning altogether.

New test under test/mocha/minify.js won't fail on master, but will fail on harmony without the fix in this PR. When this PR is merged on top of #1509, all test/compress tests pass.

[kzc]

@alexlamsl The fix looks good. Thanks.

Any idea why this problem did not occur on master without this PR?

Do you plan to apply this PR to master as well?

[alexlamsl]

The PR is targetted at master - I plan to merge master onto #1509 after this.

I think it's down to the fact that on master when has_side_effects() is called in the tests, it will eventually eliminate the side-effect-free AST_Node, so it all works out okay. But that isn't necessarily the case, and looks like something on harmony did exactly that.

There are many places where has_side_effects() is called but the expression is retained even on master, though those sites are normally not where you'd put an IIFE. I will do another check across all call sites just to make myself feel more comfortable, and then I'll merge this.

[alexlamsl]

The new tests should cover all WARN: Dropping __PURE__ call cases by now.