[TTI] Account for FMA opportunities in getArithmeticInstrCost.

[fhahn]

Summary

TTI-based alternative to D132872.

Depends on D132967.

[ABataev]

llvm/lib/Analysis/TargetTransformInfo.cpp

803

Use TTI::TCC_Free

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

6470

== TTI::TCC_Free

[fhahn]

Address latest comments, also add early exit to lambda to reduce indent, compare against FAdd cost as suggested by @dmgreen.

[fhahn]

llvm/lib/Analysis/TargetTransformInfo.cpp

803

Fixed, thanks!

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

6470

Fixed, thanks!