Run multiple iterations of JSDCE, as more may be removable

[kripken]

After we remove a function, it may enable removal of more functions.

Still no full cycle analysis etc. here, so this isn't as good as closure compiler, but it helps a little when not using closure.

Helps #5794

[kripken]

Re-did this. It now does a much better job:

• Run JSDCE with or without closure. It helps even if closure runs later. As we improve it further it may help more.
• Add AJSDCE (aggressive JSDCE) which runs multiple iterations. This is enabled on -Os, -Oz builds.
• Clean up how we add closure testing to tests in test_core.py, and add it to some -Os tests too, not just -O2, for more coverage.