[IslNodeBuilder] [NFC] Refactor creation of loop induction variables of loops outside scops.

 This logic is duplicated, so we refactor it into a separate function.
 This will be used in a later patch to teach PPCGCodeGen code generation
 for loops that are outside the scop.

 Differential Revision: https://reviews.llvm.org/D36310

llvm-svn: 310192