Reland https://codereview.chromium.org/2373983004/, reverted in
4e5a4d93.

Reason: CL is not responsible for Win32 Debug failures (see https://build.chromium.org/p/client.v8/builders/V8%20Win32%20-%20debug/builds/5031)

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Implement the logic for StringIterator.prototype.next in the JSBuiltinReducer in order to allow inlining when the receiver is a JS_STRING_ITERATOR_TYPE map, built ontop of the SimplifiedOperators StringCharCodeAt and the newly added StringFromCodePoint.

Also introduces a new StringFromCodePoint simplified op which may be useful for other String builtins, such as String.fromCodePoint()

BUG=v8:5388
TBR=bmeurer@chromium.org, mstarzinger@chromium.org

Review-Url: https://codereview.chromium.org/2394823003
Cr-Commit-Position: refs/heads/master@{#40008}