[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

llvm-svn: 370368