[RISCV] Support umin/umax in tryFoldSelectIntoOp (#157548)

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

@@ -18839,6 +18839,8 @@ static SDValue tryFoldSelectIntoOp(SDNode *N, SelectionDAG &DAG,
   case ISD::ADD:
   case ISD::OR:
   case ISD::XOR:
+  case ISD::UMIN:
+  case ISD::UMAX:
     break;
   }