[Polly] Reject regions entered by an indirectbr/callbr.

SplitBlockPredecessors is unable to insert an additional BasicBlock
between an indirectbr/callbr terminator and the successor blocks.
This is needed by Polly to normalize the control flow before emitting
its optimzed code.

This patches rejects regions entered by an indirectbr/callbr to not fail
later at code generation.

This fixes llvm.org/PR51964

Recommit with "REQUIRES: asserts" in test that uses statistics.