Hoisting fixes (#2606)

[lye]

When hoisting a var, use UNKNOWN_EXPRESSION as the initializer instead
of the actual one. This prevents the initializer from being used during
the optimization step when it hasn't yet been assigned at runtime.

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

#2606

Description

The bug is described in #2606. I more-or-less copied the examples as new test cases.

I'm not able to run the watch tests on my FreeBSD box, but the rest of npm test runs without error post-patch. The codebase was kind of hard for me to follow so grains of salt.

The patch is also sub-optimal in that it prevents the optimizer from figuring out that hoisted variables from trimmed branches are falsy, but I couldn't work out a way to plumb that through.

[nifgraup]

Confirming 5739cbd fixes the test case in #2606. (and that this test https://github.com/terser-js/terser/blob/master/test/compress/issue-2871.js passes when terser is bundled with rollup).

[lukastaegert]

Nice, thanks a lot! I was afraid that such a fix might break tree-shaking optimizations in some places but apparently this was not the case as all tests remain green :)