[ScopBuilder] Remove superfluous while loop in buildDomains. NFC.

The while loop iterating parent loop in ScopBuilder::buildDomains is
unnecessary because either L or LD are later unused, this is a simple
patch removing it.

Patch by bin.narwal <bin.narwal@gmail.com>

Differential Revision: https://reviews.llvm.org/D66698

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@370368 91177308-0d34-0410-b5e6-96231b3b80d8