[ValueTracking] recognize even more variants of smin/smax

[ValueTracking] recognize even more variants of smin/smax

Similar to:
https://reviews.llvm.org/rL285499
https://reviews.llvm.org/rL286318

We can't minimally expose this in IR tests because we don't have min/max intrinsics,
but the difference is visible in codegen because SelectionDAGBuilder::visitSelect()
uses matchSelectPattern().

We're not canonicalizing these patterns in IR (yet), so I don't expect there to be any
regressions as noted here:
http://lists.llvm.org/pipermail/llvm-dev/2016-November/106868.html

llvm-svn: 286776