[RISCV] Remove one use restriction on the scalar from combineBinOpToReduce.

[craig.topper]

Summary

If multiple reductions use the neutral start value, this prevented
us from optimizing any of them.

This increases the number of scalar->vector operations but reduces
the number of scalar operations.

Note, we were already inconsistent about the one use check when we
peeked through INSERT_SUBVECTORs.

[fakepaper56]

LGTM.

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

if (!isNonZeroAVL(ScalarV.getOperand(2)))
     return SDValue();
 
   // Check the scalar of ScalarV is neutral element
   // TODO: Deal with value other than neutral element.
   if (!isNeutralConstant(N->getOpcode(), N->getFlags(), ScalarV.getOperand(1),
                          0))
     return SDValue();
 
-  if (!ScalarV.hasOneUse())
-    return SDValue();
-
   // If the AVL is zero, operand 0 will be returned. So it's not safe to fold.
   // FIXME: We might be able to improve this if operand 0 is undef.
   if (!isNonZeroAVL(Reduce.getOperand(5)))
     return SDValue();
 
   SDValue NewStart = N->getOperand(1 - ReduceIdx);
 
   SDLoc DL(N);

llvm/test/CodeGen/RISCV/rvv/fold-binary-reduce.ll

entry:
   store float %rdx, ptr %rdxptr
   ret float %res
 }
 
 define float @reduce_fadd4(float %x, float %y, <4 x float> %v, <4 x float> %w) {
 ; CHECK-LABEL: reduce_fadd4:
 ; CHECK:       # %bb.0: # %entry
 ; CHECK-NEXT:    vsetivli zero, 4, e32, m1, ta, ma
-; CHECK-NEXT:    vmv.s.x v10, zero
+; CHECK-NEXT:    vfmv.s.f v10, fa0
 ; CHECK-NEXT:    vfredusum.vs v8, v8, v10
 ; CHECK-NEXT:    vfmv.f.s fa5, v8
-; CHECK-NEXT:    vfredusum.vs v8, v9, v10
+; CHECK-NEXT:    vfmv.s.f v8, fa1
+; CHECK-NEXT:    vfredusum.vs v8, v9, v8
 ; CHECK-NEXT:    vfmv.f.s fa4, v8
-; CHECK-NEXT:    fadd.s fa5, fa5, fa0
-; CHECK-NEXT:    fadd.s fa4, fa4, fa1
 ; CHECK-NEXT:    fdiv.s fa0, fa5, fa4
 ; CHECK-NEXT:    ret
 entry:
   %rdx = call fast float @llvm.vector.reduce.fadd.v4f32(float -0.0, <4 x float> %v)
   %rdx2 = call fast float @llvm.vector.reduce.fadd.v4f32(float -0.0, <4 x float> %w)
   %res = fadd fast float %rdx, %x
   %res2 = fadd fast float %rdx2, %y
   %div = fdiv fast float %res, %res2