[ValueTracking] recognize more variants of smin/smax

Try harder to detect obfuscated min/max patterns: the initial pattern was added with D9352 / rL236202. 
There was a bug fix for PR27137 at rL264996, but I think we can do better by folding the corresponding
smax pattern and commuted variants.

The codegen tests demonstrate the effect of ValueTracking on the backend via SelectionDAGBuilder. We
can't expose these differences minimally in IR because we don't have smin/smax intrinsics for IR.

Differential Revision: https://reviews.llvm.org/D26091

llvm-svn: 285499