[FuncSpec] Split the specialization bonus into CodeSize and Latency.

Currently we use a combined metric TargetTransformInfo::TCK_SizeAndLatency
when estimating the specialization bonus. This is suboptimal, and in some
cases erroneous. For example we shouldn't be weighting the codesize decrease
attributed to constant propagation by the block frequency of the dead code.
Instead only the latency savings should be weighted by block frequency. The
total codesize savings from all the specialization arguments should be
deducted from the specialization cost.

Differential Revision: https://reviews.llvm.org/D155103