[RISCV] Add TargetLowering::hasConditionalZero()

Will guide lowering chioces in DAGCombine.

Author: bababuck

llvm/include/llvm/CodeGen/TargetLowering.h

@@ -2502,6 +2502,10 @@ class LLVM_ABI TargetLoweringBase {
       Action != TypeSplitVector;
   }
 
+  // Return true is targets has a conditional zero-ing instruction
+  // i.e. select cond, x, 0
+  virtual bool hasConditionalZero() const { return false; }
+
   virtual bool isProfitableToCombineMinNumMaxNum(EVT VT) const { return true; }
 
   /// Return true if a select of constants (select Cond, C1, C2) should be

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -2763,6 +2763,10 @@ bool RISCVTargetLowering::mergeStoresAfterLegalization(EVT VT) const {
          (VT.isFixedLengthVector() && VT.getVectorElementType() == MVT::i1);
 }
 
+bool RISCVTargetLowering::hasConditionalZero() const {
+  return Subtarget.hasStdExtZicond() || Subtarget.hasVendorXVentanaCondOps();
+}
+
 bool RISCVTargetLowering::isLegalElementTypeForRVV(EVT ScalarTy) const {
   if (!ScalarTy.isSimple())
     return false;

llvm/lib/Target/RISCV/RISCVISelLowering.h

@@ -612,6 +612,8 @@ class RISCVTargetLowering : public TargetLowering {
     return false;
   }
 
+  bool hasConditionalZero() const override;
+
   /// Disables storing and loading vectors by default when there are function
   /// calls between the load and store, since these are more expensive than just
   /// using scalars