LLVM Clang 3.4 is still months away from being complete but already it's enabled the SLP Vectorizer, gained AArch64 NEON support, expands Loop Vectorizer improvements, and both the LLVM compiler infrastructure and Clang C/C++ compiler front-end have an assortment of other improvements.
The short answer to whether there's any major Intel Haswell gains at the moment for LLVM Clang 3.3 vs. 3.4 SVN is that there's not. Using the latest SVN code from 18 August on this System76 laptop updated to Ubuntu 13.10 is that there's very little change over 3.3 at the moment.
These very early benchmark results can be found in full at 1308197-SO-LLVM34SVN72. Full system details, logs, and all the used benchmarks can be found there with the CFLAGS/CXXFLAGS and both 3.3 and 3.4 SVN were built from source in a Release/Optimized mode. Embedded below are a few of the "interesting" results.