Fix ICE with SLP schedule at explicit region start
Correctly handles SLP scheduling at region starts to prevent ICEs.
This commit addresses an ICE (Internal Compiler Error) that occurred during SLP (Sliding Window Parallelization) scheduling at the start of explicit regions. The issue stemmed from using a random statement as a marker, which could be a control statement. The fix replaces this with NULL, adhering to documentation and resolving the ICE. A new test case is included.
In Details
The SLP vectorizer in tree-vect-slp.cc used a statement as a marker for scheduling at a region start. If this statement was a control flow statement, it could lead to an ICE. The fix ensures that only NULL is used as a marker for region starts, as per the documented interface, preventing invalid states and compiler crashes.