[LPM] Fix a logic error in LICM spotted by inspection.

We completely skipped promotion in LICM if the loop has a preheader or
dedicated exits, but not *both*. We hoist if there is a preheader, and
sink if there are dedicated exits, but either hoisting or sinking can
move loop invariant code out of the loop!

I have no idea if this has a practical consequence. If anyone has ideas
for a test case, let me know.