As test in PR56672 shows, LAA produces different results which lead to either
positive or negative vectorization decisions depending on the order of blocks
in loop. The exact reason of this is not clear to me, however this makes investigation
of related bugs extremely complex.
Current order of blocks in the loop is arbitrary. It may change, for example, if loop
info analysis is dropped and recomputed. Seems that it interferes with LAA's logic.
This patch chooses fixed traversal order of blocks in loops, making it RPOT.
Note: this is *not* a fix for bug with incorrect analysis result. It just makes
the answer more robust to make the investigation easier.
Differential Revision: https://reviews.llvm.org/D130482
Reviewed By: aeubanks, fhahn
Max Kazantsev