Use faster cached regexps for tests in setInnerHTML

This is significantly faster than the current code.
http://jsperf.com/reacttestindexof (only relevant on IE8)

Also added comments to the list of non-visible tags and made the test even stricter.

Since this code is explicitly aimed at IE8, we could also switch the current extensive "feature test" for just `document.documentMode === 8`, it correctly identifies emulation mode (which emulates the crappy whitespace behavior). But it's hard to guarantee that there isn't some weird edge-case where it could differ.