Aggressive FMA fusion for NVPTX

Hi,

I propose to override the TLI callback enableAggressiveFMAFusion for the NVPTX backend and return true instead of false. The reason is the same as for PPC: fmul, fmadd and fadd nodes cost the same number of cycles (see http://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#arithmetic-instructions), so we can enable more combining heuristics to produce more FMAs. For instance, this pattern would be considered:

// fold (fadd (fma x, y, (fmul u, v)), z) → (fma x, y (fma u, v, z))

cf. commits:
http://llvm.org/viewvc/llvm-project?view=revision&revision=218120
http://llvm.org/viewvc/llvm-project?view=revision&revision=225380

Please tell me what you think.

Olivier

Looks good to me! Thanks!