Patch forthcoming. This will take some iterating to get right.
Created attachment 213652 [details] work in progress
Created attachment 213707 [details] more!
<rdar://problem/15183189>
Leaving this open until the LLVM side of the patches lands and we can enable this by default.
This works reliably as of http://trac.webkit.org/changeset/157382, but I'll leave it open until the functionality is available in LLVM trunk.
It's in trunk. The remaining work to port the implementation to the slight changes between the original LLVM patch and what landed will be covered in https://bugs.webkit.org/show_bug.cgi?id=122318.