[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 that
do lowerSELECT as before when foldBinOpIntoSelectIfProfitable returns a select node,
and return the node directly when foldBinOpIntoSelectIfProfitable returns a constant node.

Author: Yunzezhu94

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -7664,7 +7664,11 @@ 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);
+        // Opcode check is necessary because foldBinOpIntoSelectIfProfitable
+        // may return a constant node and cause crash in lowerSELECT.
+        if (NewSel.getOpcode() == ISD::SELECT)
+          return lowerSELECT(NewSel, DAG);
+        return NewSel;
       }
     }
   }

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

@@ -0,0 +1,14 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+; 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
+}