[RISCV] Add a check in lowerSELECT after foldBinOpIntoSelectIfProfitable

In certain case foldBinOpIntoSelectIfProfitable may return a constant node,
the node will be lowered in lowerSELECT and lead to crash.
This patch fix the bug by adding an extra check before lowerSELECT.

Author: Yunzezhu94

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -7664,7 +7664,10 @@ SDValue RISCVTargetLowering::lowerSELECT(SDValue Op, SelectionDAG &DAG) const {
       if (SDValue NewSel = foldBinOpIntoSelectIfProfitable(*Op->use_begin(),
                                                            DAG, Subtarget)) {
         DAG.ReplaceAllUsesWith(BinOp, &NewSel);
-        return lowerSELECT(NewSel, DAG);
+        if (NewSel.getOpcode() == ISD::SELECT)
+          return lowerSELECT(NewSel, DAG);
+        else
+          return NewSel;
       }
     }
   }

llvm/test/CodeGen/RISCV/fold-binop-into-select-return-constant.ll

@@ -0,0 +1,13 @@
+; RUN: llc -mtriple=riscv64 -mattr=+m < %s  | FileCheck  %s
+
+define i64 @fold_binop_into_select_return_constant(i1 %c) {
+; CHECK-LABEL: fold_binop_into_select_return_constant:
+; CHECK:       # %bb.0: # %entry
+; CHECK-NEXT:    li a0, 0
+; CHECK-NEXT:    ret
+entry:
+  %select1 = select i1 %c, i32 4, i32 8
+  %select2 = sext i32 %select1 to i64
+  %div1 = sdiv i64 %select2, -5141143369814759789
+  ret i64 %div1
+}