[Polly] Remove unwanted --check-prefix=CHECK from unit tests. NFC.

Summary:
Removed unwanted --check-prefix=CHECK from the following unit tests:
      DeadCodeElimination/dead_iteration_elimination.ll
      Isl/CodeGen/simple_vec_cast.ll

Patch by: Mandeep Singh Grang (mgrang)

Reviewers: jdoerfert, zinob, spop, grosser

Projects: #polly

Differential Revision: http://reviews.llvm.org/D19143

llvm-svn: 266411