[SelectionDAG] Add SPLAT_VECTOR to SelectionDAG::isConstantFPBuildVectorOrConstantFP.

[craig.topper]

Summary

Matches what is done for the int version.

[craig.topper]

Add test case for simplifying fmul with 1.0.

[sdesmalen]

Thanks, LGTM!

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

 //===- SelectionDAG.cpp - Implement the SelectionDAG data structures ------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //

 // or constant float.
 SDNode *SelectionDAG::isConstantFPBuildVectorOrConstantFP(SDValue N) const {
   if (isa<ConstantFPSDNode>(N))
     return N.getNode();
 
   if (ISD::isBuildVectorOfConstantFPSDNodes(N.getNode()))
     return N.getNode();
 
+  if ((N.getOpcode() == ISD::SPLAT_VECTOR) &&
+      isa<ConstantFPSDNode>(N.getOperand(0)))
+    return N.getNode();
+
   return nullptr;
 }
 
 void SelectionDAG::createOperands(SDNode *Node, ArrayRef<SDValue> Vals) {
   assert(!Node->OperandList && "Node already has operands");
   assert(SDNode::getMaxNumOperands() >= Vals.size() &&
          "too many operands to fit into SDNode");
   SDUse *Ops = OperandRecycler.allocate(

llvm/test/CodeGen/RISCV/rvv/combine-splats.ll

; CHECK-NEXT:    ret
   %ins1 = insertelement <vscale x 8 x i16> poison, i16 2, i32 0
   %splat1 = shufflevector <vscale x 8 x i16> %ins1, <vscale x 8 x i16> poison, <vscale x 8 x i32> zeroinitializer
   %ins2 = insertelement <vscale x 8 x i16> poison, i16 4, i32 0
   %splat2 = shufflevector <vscale x 8 x i16> %ins2, <vscale x 8 x i16> poison, <vscale x 8 x i32> zeroinitializer
   %v1 = lshr <vscale x 8 x i16> %x, %splat2
   %v2 = lshr <vscale x 8 x i16> %v1, %splat2
   ret <vscale x 8 x i16> %v2
 }
+
+; fold (fmul x, 1.0) -> x
+define <vscale x 2 x float> @combine_fmul_one(<vscale x 2 x float> %x) {
+; CHECK-LABEL: combine_fmul_one:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    ret
+  %ins = insertelement <vscale x 2 x float> poison, float 1.0, i32 0
+  %splat = shufflevector <vscale x 2 x float> %ins, <vscale x 2 x float> poison, <vscale x 2 x i32> zeroinitializer
+  %v = fmul <vscale x 2 x float> %x, %splat
+  ret <vscale x 2 x float> %v
+}
+
+; fold (fmul 1.0, x) -> x
+define <vscale x 2 x float> @combine_fmul_one_commuted(<vscale x 2 x float> %x) {
+; CHECK-LABEL: combine_fmul_one_commuted:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    ret
+  %ins = insertelement <vscale x 2 x float> poison, float 1.0, i32 0
+  %splat = shufflevector <vscale x 2 x float> %ins, <vscale x 2 x float> poison, <vscale x 2 x i32> zeroinitializer
+  %v = fmul <vscale x 2 x float> %splat, %x
+  ret <vscale x 2 x float> %v
+}