[TTI] Let the cost model estimate ctpop costs based on legality

[TTI] Let the cost model estimate ctpop costs based on legality

PPC has a vector popcount, this lets the vectorizer use the correct cost
for it. Tweak X86 test to use an intrinsic that's actually scalarized (we
have a somewhat efficient lowering for vector popcount using SSE, the
cost model finds that now).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@265005 91177308-0d34-0410-b5e6-96231b3b80d8