When analyzing vectors of element type that require legalization,
the legalization cost must be included to get an accurate
estimation of the total cost of the scalarized vector.
The inaccurate cost triggered unprofitable SLP vectorization on
32-bit X86.

Summary:
Include legalization overhead when computing scalarization cost

Reviewers: hfinkel, nadav

CC: chandlerc, rnk, llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2992