independent blocks: do not insert stores between phi nodes

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@182661 91177308-0d34-0410-b5e6-96231b3b80d8
add comments to clarify the use of a temporary variable in the map insertion

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@182662 91177308-0d34-0410-b5e6-96231b3b80d8