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[DAGCombine] Check the uses of negated floating constant and remove the hack

Authored by steven.zhang on Mar 4 2020, 7:42 PM.

Description

[DAGCombine] Check the uses of negated floating constant and remove the hack

PowerPC hits an assertion due to somewhat the same reason as https://reviews.llvm.org/D70975.
Though there are already some hack, it still failed with some case, when the operand 0 is NOT
a const fp, it is another fma that with const fp. And that const fp is negated which result in multi-uses.

A better fix is to check the uses of the negated const fp. If there are already use of its negated
value, we will have benefit as no extra Node is added.

Differential revision: https://reviews.llvm.org/D75501