[RISCV] Support umin/umax in tryFoldSelectIntoOp

The neutral values for these are -1U, and 0 respectively.  We already
have good arithmetic lowerings for selects with one arm equal to these
values.  smin/smax are a bit harder, and will be a separate change.

Somewhat surprisingly, this looks to be a net code improvement in all
of the configurations.  With both zbb, it's a clear win.  With only
zicond, we still seem to come out ahead because we reduce the
number of ziconds needed (since we lower min/max to them).  Without
either zbb or zicond, we're a bit more of wash, but the available
arithmetic sequences are good enough that doing the select
unconditionally before using branches for the min/max is probably
still worthwhile?

Author: preames

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -18835,6 +18835,8 @@ static SDValue tryFoldSelectIntoOp(SDNode *N, SelectionDAG &DAG,
   case ISD::ADD:
   case ISD::OR:
   case ISD::XOR:
+  case ISD::UMIN:
+  case ISD::UMAX:
     break;
   }