[llvm] [MachinePipeliner] Improve loop carried dependence analysis (PR #94185)

[Yuta Mukai via llvm-commits]

ytmukai wrote:

@quic-santdas Sorry for the late reply. Thank you for your review! 

I counted the number of loops pipelined in llvm-test-suite for Hexagon. The results are as follows.

| Both pipelined | Neither pipelined | Pipelined only in the current code | Pipelined only with this patch |
| ---:| ---:| ---:| ---:|
| 6944 | 10867 | 84 | 75 |

There are cases where this patch enables pipelining as a result of unnecessary dependencies being removed.
As for the cases that could no longer be pipelined, many of them seemed to have happened to be unabled due to changes in the DDG.

The number of loops affected is small, so applying this patch does not seem to cause major problems.
Could someone please approve this patch? 

https://github.com/llvm/llvm-project/pull/94185