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[FPEnv] Don't transform FSUB(-0,X)->FNEG(X) in SelectionDAGBuilder.

Authored by cameron.mcinally on Aug 3 2020, 8:19 AM.

Description

[FPEnv] Don't transform FSUB(-0,X)->FNEG(X) in SelectionDAGBuilder.

This patch stops unconditionally transforming FSUB(-0,X) into an FNEG(X) while building the DAG. There is also one small change to handle the new FSUB(-0,X) similarly to FNEG(X) in the AMDGPU backend.

Differential Revision: https://reviews.llvm.org/D84056