[SelectionDAG] Don't remove unused negated constant immediately

This reverts partial of a2fb544 (actually, 2508ef0) about removing
negated FP constant immediately if it has no uses. However, as discussed
in bug 47517, there're cases when NegX is folded into constant from
other places while NegY is removed by that line of code and NegX is
equal to NegY. In these cases, NegX is deleted before used and crash
happens. So revert the code and add necessary test case.

(cherry picked from commit b326d4f)

Author: ecnelises

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -5751,10 +5751,8 @@ SDValue TargetLowering::getNegatedExpression(SDValue Op, SelectionDAG &DAG,
 
     // If we already have the use of the negated floating constant, it is free
     // to negate it even it has multiple uses.
-    if (!Op.hasOneUse() && CFP.use_empty()) {
-      RemoveDeadNode(CFP);
+    if (!Op.hasOneUse() && CFP.use_empty())
       break;
-    }
     Cost = NegatibleCost::Neutral;
     return CFP;
   }

llvm/test/CodeGen/X86/pr47517.ll

@@ -26,3 +26,16 @@ entry:
   %fmul6 = fmul fast float %fmul3, %fadd4
   ret float %fmul6
 }
+
+; To ensure negated result will not be removed when NegX=NegY and
+; NegX is needed
+define float @test2(float %x, float %y) {
+  %add = fadd fast float %x, 750.0
+  %sub = fsub fast float %x, %add
+  %mul = fmul fast float %sub, %sub
+  %mul2 = fmul fast float %mul, %sub
+  %add2 = fadd fast float %mul2, 1.0
+  %add3 = fadd fast float %mul2, %add2
+  %mul3 = fmul fast float %y, %add3
+  ret float %mul3
+}