Newly created fneg was built after some of it's uses in some cases.
Now it will be built immediately after instruction whose dst it negates.
Differential Revision: https://reviews.llvm.org/D119459
If possible fold fneg into instruction above if users cannot fold mods and we
know it will decrease instruction count.
Follows same logic as SDAG combiner in choosing opportunities to combine.
Differential Revision: https://reviews.llvm.org/D112827
