[InstSimplify] fixed (?) to not mutate icmps

As Eli noted in the post-commit thread for r288833, the use of
swapOperands() may not be allowed in InstSimplify, so I'm 
removing those calls here pending further review. 

The swap mutates the icmp, and there doesn't appear to be precedent
for instruction mutation in InstSimplify.

I didn't actually have any tests for those cases, so I'm adding
a few here. 

llvm-svn: 288855