use -0.0 when creating an fneg instruction

[spatel]

Summary

Simple (I hope) bug fix when generating an fneg operation in InstCombine. Backends recognize (-0.0 - X) as the canonical form for fneg and produce better code. Eg, ppc64 with 0.0:

lis r2, ha16(LCPI0_0)
lfs f0, lo16(LCPI0_0)(r2)
fsubs f1, f0, f1
blr

Vs. -0.0:

fneg f1, f1
blr

llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp

 Value *FAddCombine::createFSub(Value *Opnd0, Value *Opnd1) {
   Value *V = Builder->CreateFSub(Opnd0, Opnd1);
   if (Instruction *I = dyn_cast<Instruction>(V))
     createInstPostProc(I);
   return V;
 }
 
 Value *FAddCombine::createFNeg(Value *V) {
-  Value *Zero = cast<Value>(ConstantFP::get(V->getType(), 0.0));
+  Value *Zero = cast<Value>(ConstantFP::getZeroValueForNegation(V->getType()));
   Value *NewV = createFSub(Zero, V);
   if (Instruction *I = dyn_cast<Instruction>(NewV))
     createInstPostProc(I, true); // fneg's don't receive instruction numbers.
   return NewV;
 }
 
 Value *FAddCombine::createFAdd(Value *Opnd0, Value *Opnd1) {
   Value *V = Builder->CreateFAdd(Opnd0, Opnd1);

llvm/trunk/test/Transforms/InstCombine/fast-math.ll

 
 ; X - (X + Y) => 0 - Y
 define float @fold9(float %f1, float %f2) {
   %t1 = fadd float %f1, %f2
   %t3 = fsub fast float %f1, %t1
   ret float %t3
 
 ; CHECK-LABEL: @fold9(
-; CHECK: fsub fast float 0.000000e+00, %f2
+; CHECK: fsub fast float -0.000000e+00, %f2
 }
 
 ; Let C3 = C1 + C2. (f1 + C1) + (f2 + C2) => (f1 + f2) + C3 instead of
 ; "(f1 + C3) + f2" or "(f2 + C3) + f1". Placing constant-addend at the
 ; top of resulting simplified expression tree may potentially reveal some
 ; optimization opportunities in the super-expression trees.
 ;
 define float @fold10(float %f1, float %f2) {