[RISCV] Use source element type for the zero constant in IR for vwcvt intrinsic. (#148023)

The vwcvt intrinsic produces a vwadd with a scalar 0 for the RHS. We
should be using the element type of the source so that the 0 needs to be
widened.

The i32->i64 vwcvt previously failed on RV32 because the legalization
code doesn't expect to see an i64 type.

Author: topperc

clang/include/clang/Basic/riscv_vector.td

@@ -499,7 +499,7 @@ multiclass RVVPseudoVWCVTBuiltin<string IR, string MName, string type_range,
           if (PolicyAttrs & RVV_VTA)
             Ops.insert(Ops.begin(), llvm::PoisonValue::get(ResultType));
         }
-        auto ElemTy = cast<llvm::VectorType>(ResultType)->getElementType();
+        auto ElemTy = cast<llvm::VectorType>(Ops[1]->getType())->getElementType();
         Ops.insert(Ops.begin() + 2, llvm::Constant::getNullValue(ElemTy));
         if (IsMasked) {
           Ops.push_back(ConstantInt::get(Ops.back()->getType(), PolicyAttrs));