[TTI] The cost model should not assume vector casts get completely scalarized

The cost model should not assume vector casts get completely scalarized, since
on targets that have vector support, the common case is a partial split up to
the legal vector size. So, when a vector cast  gets split, the resulting casts
end up legal and cheap.

Instead of pessimistically assuming scalarization, base TTI can use the costs
the concrete TTI provides for the split vector, plus a fudge factor to account
for the cost of the split itself. This fudge factor is currently 1 by default,
except on AMDGPU where inserts and extracts are considered free.

Differential Revision: http://reviews.llvm.org/D21251

llvm-svn: 274642