[RISCV] Reduce code duplication. NFC (#171577)

This code was only different in the opcodes of the nodes it created.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -9786,22 +9786,17 @@ SDValue RISCVTargetLowering::lowerSELECT(SDValue Op, SelectionDAG &DAG) const {
       SDValue ConstVal = IsCZERO_NEZ ? TrueV : FalseV;
       SDValue RegV = IsCZERO_NEZ ? FalseV : TrueV;
       int64_t RawConstVal = cast<ConstantSDNode>(ConstVal)->getSExtValue();
-      // Fall back to XORI if Const == -0x800
-      if (RawConstVal == -0x800) {
-        SDValue XorOp = DAG.getNode(ISD::XOR, DL, VT, RegV, ConstVal);
-        SDValue CMOV =
-            DAG.getNode(IsCZERO_NEZ ? RISCVISD::CZERO_NEZ : RISCVISD::CZERO_EQZ,
-                        DL, VT, XorOp, CondV);
-        return DAG.getNode(ISD::XOR, DL, VT, CMOV, ConstVal);
-      }
       // Efficient only if the constant and its negation fit into `ADDI`
       // Prefer Add/Sub over Xor since can be compressed for small immediates
       if (isInt<12>(RawConstVal)) {
-        SDValue SubOp = DAG.getNode(ISD::SUB, DL, VT, RegV, ConstVal);
-        SDValue CMOV =
+        // Fall back to XORI if Const == -0x800 since we don't have SUBI.
+        unsigned SubOpc = (RawConstVal == -0x800) ? ISD::XOR : ISD::SUB;
+        unsigned AddOpc = (RawConstVal == -0x800) ? ISD::XOR : ISD::ADD;
+        SDValue SubOp = DAG.getNode(SubOpc, DL, VT, RegV, ConstVal);
+        SDValue CZERO =
             DAG.getNode(IsCZERO_NEZ ? RISCVISD::CZERO_NEZ : RISCVISD::CZERO_EQZ,
                         DL, VT, SubOp, CondV);
-        return DAG.getNode(ISD::ADD, DL, VT, CMOV, ConstVal);
+        return DAG.getNode(AddOpc, DL, VT, CZERO, ConstVal);
       }
     }