[InstCombine] return when SimplifyAssociativeOrCommutative makes a change

This bug was created by rL335258 because we used to always call instsimplify
after trying the associative folds. After that change it became possible
for subsequent folds to encounter unsimplified code (and potentially assert
because of it). 

Instead of carrying changed state through instcombine, we can just return 
immediately. This allows instsimplify to run, so we can continue assuming
that easy folds have already occurred.

llvm-svn: 336965