Implement sext(C1 + C2*X) --> sext(C1) + sext(C2*X) and
sext{C1,+,C2} --> sext(C1) + sext{0,+,C2} transformation in Scalar
Evolution.

That helps SLP-vectorizer to recognize consecutive loads/stores.

<rdar://problem/14860614>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@209568 91177308-0d34-0410-b5e6-96231b3b80d8