[InstCombine] add tests to show missed canonicalization of min/max; NFC

Unfortunately, recognizing these in value tracking may cause us to hit
a hack in InstCombiner::visitICmpInst() more often:
http://lists.llvm.org/pipermail/llvm-dev/2017-January/109340.html

...but besides being the obviously Right Thing To Do, there's a clear 
codegen win from identifying these patterns for several targets.

llvm-svn: 292655