[Acc] Fix for PR33208

During codegen, Polly attempts to clear all loops from ScalarEvolution
and LoopInfo, and it does so one block at a time. This causes undefined
behaviour, since this way a loop header might be removed from a loop
before the entire loop is erased, causing ScalarEvolution to run into an
error.

Instead, just delete the entire loop atomically. This fixes currently
failing testcases.

llvm-svn: 326643