[VectorCombine] check instruction type before dispatching to folds

This is no externally visible change intended, but appears to be a
noticeable (surprising) improvement in compile-time based on:
https://llvm-compile-time-tracker.com/compare.php?from=0f3e72e86c8c7c6bf0ec24bf1e2acd74b4123e7b&to=5e8c2026d10e8e2c93c038c776853bed0e7c8fc1&stat=instructions:u

The early returns in the individual fold functions are not good
enough to avoid the overhead of the many "fold*" calls, so this
speeds up the main instruction loop enough to make a difference.