codegen: Update LoopInfo correctly

Add the 'polly.start' basic block to the loop that surrounds the scop we just
codegenerate.

This fixes PR13441

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@202000 91177308-0d34-0410-b5e6-96231b3b80d8