[InstCombine] canonicalize non-obivous forms of integer min/max

This is part of trying to clean up our handling of min/max patterns in IR.
By converting these to canonical form, we're more likely to recognize them
because there are various places in InstCombine that don't use 
matchSelectPattern or m_SMax and friends.

The backend fixups referenced in the now deleted TODO comment were added with:
https://reviews.llvm.org/rL291392
https://reviews.llvm.org/rL289738

If there's any codegen fallout from this change, we should be able to address
it in DAGCombiner or target-specific lowering. 

llvm-svn: 295758