[TargetLowering][RISCV] Allow truncation when checking if the argumen…

…ts of a setcc are splats.

We're just trying to canonicalize here and won't be using the constant
value returned.

The attached test changes are because we were previously commuting
a seteq X, (splat_vector 0) because we also have (sub 0, X). The
0 is larger than the element type so we don't detect it as a splat
without the AllowTruncation flag. By preventing the commute we are
able to match it to the vmseq.vx instruction during isel. We only
look for constants on the RHS in isel.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D123256

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -3955,13 +3955,17 @@ SDValue TargetLowering::SimplifySetCC(EVT VT, SDValue N0, SDValue N1,
   if (SDValue Fold = DAG.FoldSetCC(VT, N0, N1, Cond, dl))
     return Fold;
 
+  bool N0ConstOrSplat =
+      isConstOrConstSplat(N0, /*AllowUndefs*/ false, /*AllowTruncate*/ true);
+  bool N1ConstOrSplat =
+      isConstOrConstSplat(N1, /*AllowUndefs*/ false, /*AllowTruncate*/ true);
+
   // Ensure that the constant occurs on the RHS and fold constant comparisons.
   // TODO: Handle non-splat vector constants. All undef causes trouble.
   // FIXME: We can't yet fold constant scalable vector splats, so avoid an
   // infinite loop here when we encounter one.
   ISD::CondCode SwappedCC = ISD::getSetCCSwappedOperands(Cond);
-  if (isConstOrConstSplat(N0) &&
-      (!OpVT.isScalableVector() || !isConstOrConstSplat(N1)) &&
+  if (N0ConstOrSplat && (!OpVT.isScalableVector() || !N1ConstOrSplat) &&
       (DCI.isBeforeLegalizeOps() ||
        isCondCodeLegal(SwappedCC, N0.getSimpleValueType())))
     return DAG.getSetCC(dl, VT, N1, N0, SwappedCC);
@@ -3970,7 +3974,7 @@ SDValue TargetLowering::SimplifySetCC(EVT VT, SDValue N0, SDValue N1,
   // -- but in reverse order -- then try to commute the operands of this setcc
   // to match. A matching pair of setcc (cmp) and sub may be combined into 1
   // instruction on some targets.
-  if (!isConstOrConstSplat(N0) && !isConstOrConstSplat(N1) &&
+  if (!N0ConstOrSplat && !N1ConstOrSplat &&
       (DCI.isBeforeLegalizeOps() ||
        isCondCodeLegal(SwappedCC, N0.getSimpleValueType())) &&
       DAG.doesNodeExist(ISD::SUB, DAG.getVTList(OpVT), {N1, N0}) &&