While your statement can certainly be correct for a sufficiently stringent definition of "advanced" (and replacing "instructions" with "code patterns", etc.), in my experience in using both clang for C++, Julia (a language largely reliant on the LLVM optimizer) and LDC (a D language compiler on top of the LLVM) vectorization support is competitive (and sometimes superior) to GCC. Comparisons with ICC are somewhat more complicated, ICC is unparalleled for specific patterns and often fails horribly in other things, the variance there is quite huge.

Disclaimer: by "vectorization" I'm referring to SSE4, AVX and AVX2, I haven't had a chance to try out AVX512 yet.