CalcSpillWeights: give a better describing name to calculateSpillWeights

Besides, this relates it more obviously to the VirtRegAuxInfo::calculateSpillWeightAndHint.

No functionnal change.
Improve the documentation of the optimization flags

Reviewers: rafael.espindola, rengolin, hfinkel

Reviewed By: rengolin

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2138