[MLIR][Mem2Reg] Add support for region control flow and SCF (#185036)
This PR adds support for region control-flow. Region control-flow and
CFG can be mixed together in the same program. See the [accompanying
RFC](https://discourse.llvm.org/t/rfc-support-region-control-flow-in-mem2reg/90082)
for some design considerations.
Beyond the considerations in the RFC, a few minor changes were
introduced:
- Calling the visitor hook for defined values is now deferred to the end
of promotion.
- The lazy creation of default values has been moved to the places where
it happens to prepare for a future change where it is actually lazy.
Documentation about it not working as intended for now was also added.
All SCF operations are supported, including `forall` and `parallel`,
which is pretty cool I think.
I am sorry in advance for git diff displaying a really bad diff for
Mem2Reg.cpp around where the liveness analysis used to be. Do consider
simply reading this part of the code off the file.
As a disclaimer, I designed all the test cases myself, but I used a
large amount of matrix multiplications to produce the corresponding IR
and FileCheck tests. I have reviewed them carefully and they correspond
to my intent.
---------
Co-authored-by: Slava Zakharin <szakharin@nvidia.com>
llvm-project
239ca11a - [MLIR][Mem2Reg] Add support for region control flow and SCF (#185036)
