[instcombine] remove fsub to fneg hacks; only emit fneg

Summary: Rewrite the fsub-0.0 idiom to fneg and always emit fneg for fp
negation. This also extends the scalarization cost in instcombine for unary
operators to result in the same IR rewrites for fneg as for the idiom.

Reviewed By: cameron.mcinally

Differential Revision: https://reviews.llvm.org/D75467