[InstSimplify] remove redundant folds

The 'hasOneUse' check is a giveaway that something's not right.
We never need to check that in InstSimplify because we don't
create new instructions here.

These are all handled as icmp simplifies which then trigger
existing select simplifies, so there's no need to duplicate 
a composite fold of the two.

llvm-svn: 326750