[Refactor][NFC] Simplify (Vector)BlockGenerator

  This removes an unnecessary cast and a unnecessary local variable.

Differential Revision: http://reviews.llvm.org/D5662


git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@219338 91177308-0d34-0410-b5e6-96231b3b80d8