Add SDNodes for umin, umax, smin and smax.

Add SDNodes for umin, umax, smin and smax.

This adds new SDNodes for signed/unsigned min/max. These nodes are built from
select/icmp pairs matched at SDAGBuilder stage.

This patch adds the nodes, as well as legalization support and sets them to
be "expand" for all targets.

NFC for now; this will be tested when I switch AArch64 to using these new
nodes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237423 91177308-0d34-0410-b5e6-96231b3b80d8