[RISCV] Use vwadd.vx for splat vector with extension (llvm#87249)

This patch allows `combineBinOp_VLToVWBinOp_VL` to handle patterns like
`(splat_vector (sext op))` or `(splat_vector (zext op))`. Then we can
use `vwadd.vx` and `vwadd.w` for such a case.

### Source code
```
define <vscale x 8 x i64> @vwadd_vx_splat_sext(<vscale x 8 x i32> %va, i32 %b) {
     %sb = sext i32 %b to i64
     %head = insertelement <vscale x 8 x i64> poison, i64 %sb, i32 0
     %splat = shufflevector <vscale x 8 x i64> %head, <vscale x 8 x i64> poison, <vscale x 8 x i32> zeroinitializer
     %vc = sext <vscale x 8 x i32> %va to <vscale x 8 x i64>
     %ve = add <vscale x 8 x i64> %vc, %splat
     ret <vscale x 8 x i64> %ve
}
```

### Before this patch
[Compiler Explorer](https://godbolt.org/z/sq191PsT4)
```
vwadd_vx_splat_sext:
  sext.w a0, a0
  vsetvli a1, zero, e64, m8, ta, ma
  vmv.v.x v16, a0
  vsetvli zero, zero, e32, m4, ta, ma
  vwadd.wv v16, v16, v8
  vmv8r.v v8, v16
  ret
```
### After this patch
```
vwadd_vx_splat_sext
  vsetvli a1, zero, e32, m4, ta, ma
  vwadd.vx v16, v8, a0
  vmv8r.v v8, v16
  ret
```

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -13597,7 +13597,8 @@ struct NodeExtensionHelper {
 
   /// Check if this instance represents a splat.
   bool isSplat() const {
-    return OrigOperand.getOpcode() == RISCVISD::VMV_V_X_VL;
+    return OrigOperand.getOpcode() == RISCVISD::VMV_V_X_VL ||
+           OrigOperand.getOpcode() == ISD::SPLAT_VECTOR;
   }
 
   /// Get the extended opcode.
@@ -13641,6 +13642,8 @@ struct NodeExtensionHelper {
     case RISCVISD::VZEXT_VL:
     case RISCVISD::FP_EXTEND_VL:
       return DAG.getNode(ExtOpc, DL, NarrowVT, Source, Mask, VL);
+    case ISD::SPLAT_VECTOR:
+      return DAG.getSplat(NarrowVT, DL, Source.getOperand(0));
     case RISCVISD::VMV_V_X_VL:
       return DAG.getNode(RISCVISD::VMV_V_X_VL, DL, NarrowVT,
                          DAG.getUNDEF(NarrowVT), Source.getOperand(1), VL);
@@ -13776,6 +13779,47 @@ struct NodeExtensionHelper {
   /// Check if this node needs to be fully folded or extended for all users.
   bool needToPromoteOtherUsers() const { return EnforceOneUse; }
 
+  void fillUpExtensionSupportForSplat(SDNode *Root, SelectionDAG &DAG,
+                                      const RISCVSubtarget &Subtarget) {
+    unsigned Opc = OrigOperand.getOpcode();
+    MVT VT = OrigOperand.getSimpleValueType();
+
+    assert((Opc == ISD::SPLAT_VECTOR || Opc == RISCVISD::VMV_V_X_VL) &&
+           "Unexpected Opcode");
+
+    // The pasthru must be undef for tail agnostic.
+    if (Opc == RISCVISD::VMV_V_X_VL && !OrigOperand.getOperand(0).isUndef())
+      return;
+
+    // Get the scalar value.
+    SDValue Op = Opc == ISD::SPLAT_VECTOR ? OrigOperand.getOperand(0)
+                                          : OrigOperand.getOperand(1);
+
+    // See if we have enough sign bits or zero bits in the scalar to use a
+    // widening opcode by splatting to smaller element size.
+    unsigned EltBits = VT.getScalarSizeInBits();
+    unsigned ScalarBits = Op.getValueSizeInBits();
+    // Make sure we're getting all element bits from the scalar register.
+    // FIXME: Support implicit sign extension of vmv.v.x?
+    if (ScalarBits < EltBits)
+      return;
+
+    unsigned NarrowSize = VT.getScalarSizeInBits() / 2;
+    // If the narrow type cannot be expressed with a legal VMV,
+    // this is not a valid candidate.
+    if (NarrowSize < 8)
+      return;
+
+    if (DAG.ComputeMaxSignificantBits(Op) <= NarrowSize)
+      SupportsSExt = true;
+
+    if (DAG.MaskedValueIsZero(Op,
+                              APInt::getBitsSetFrom(ScalarBits, NarrowSize)))
+      SupportsZExt = true;
+
+    EnforceOneUse = false;
+  }
+
   /// Helper method to set the various fields of this struct based on the
   /// type of \p Root.
   void fillUpExtensionSupport(SDNode *Root, SelectionDAG &DAG,
@@ -13814,43 +13858,10 @@ struct NodeExtensionHelper {
     case RISCVISD::FP_EXTEND_VL:
       SupportsFPExt = true;
       break;
-    case RISCVISD::VMV_V_X_VL: {
-      // Historically, we didn't care about splat values not disappearing during
-      // combines.
-      EnforceOneUse = false;
-
-      // The operand is a splat of a scalar.
-
-      // The pasthru must be undef for tail agnostic.
-      if (!OrigOperand.getOperand(0).isUndef())
-        break;
-
-      // Get the scalar value.
-      SDValue Op = OrigOperand.getOperand(1);
-
-      // See if we have enough sign bits or zero bits in the scalar to use a
-      // widening opcode by splatting to smaller element size.
-      MVT VT = Root->getSimpleValueType(0);
-      unsigned EltBits = VT.getScalarSizeInBits();
-      unsigned ScalarBits = Op.getValueSizeInBits();
-      // Make sure we're getting all element bits from the scalar register.
-      // FIXME: Support implicit sign extension of vmv.v.x?
-      if (ScalarBits < EltBits)
-        break;
-
-      unsigned NarrowSize = VT.getScalarSizeInBits() / 2;
-      // If the narrow type cannot be expressed with a legal VMV,
-      // this is not a valid candidate.
-      if (NarrowSize < 8)
-        break;
-
-      if (DAG.ComputeMaxSignificantBits(Op) <= NarrowSize)
-        SupportsSExt = true;
-      if (DAG.MaskedValueIsZero(Op,
-                                APInt::getBitsSetFrom(ScalarBits, NarrowSize)))
-        SupportsZExt = true;
+    case ISD::SPLAT_VECTOR:
+    case RISCVISD::VMV_V_X_VL:
+      fillUpExtensionSupportForSplat(Root, DAG, Subtarget);
       break;
-    }
     default:
       break;
     }