[VectorCombine] switch on opcode to compile faster

This follows 87debdad to further eliminate wasting time
calling helper functions only to early return to the main
run loop.

Once again, this results in significant savings based on
experimental data:
https://llvm-compile-time-tracker.com/compare.php?from=01023bfcd33f922ed8c934ce563e54abe8bfe246&to=3dce4f70b73e48ccb045decb634c185e6b4c67d5&stat=instructions:u

This is NFCI other than making the pass faster. The total
cost of VectorCombine runs in an -O3 build appears to be
well under 0.1% of compile-time now, so there's not much
left to do AFAICT.

There's a TODO about making the code cleaner, but it
probably doesn't change timing much. I didn't include those
changes here because it requires updating much more code.