This is similar to the fix to the greedy driver in #153957 ; except that
instead of removing unreachable code, we just ignore it.
Operations like:
```
%add = arith.addi %add, %add : i64
```
are legal in unreachable code.
Unfortunately many patterns would be unsafe to apply on such IR and can
lead to crashes or infinite loops.
This is in preparation of a follow-up change to stop traversing
unreachable blocks.
This is not NFC because of a subtlety of the early_inc. On a test case
like:
```
scf.if %cond {
"test.move_after_parent_op"() ({
"test.any_attr_of_i32_str"() {attr = 0 : i32} : () -> ()
}) : () -> ()
}
```
We recursively traverse the nested regions, and process an op when the
region is done (post-order).
We need to pre-increment the iterator before processing an operation in
case it gets deleted. However
we can do this before or after processing the nested region. This
implementation does the latter.
This is intended as a fast pattern rewrite driver for the cases when a
simple walk gets the job done but we would still want to implement it in
terms of rewrite patterns (that can be used with the greedy pattern
rewrite driver downstream).
The new driver is inspired by the discussion in
https://github.com/llvm/llvm-project/pull/112454 and the LLVM Dev
presentation from @matthias-springer earlier this week.
This limitation comes with some limitations:
* It does not repeat until a fixpoint or revisit ops modified in place
or newly created ops. In general, it only walks forward (in the
post-order).
* `matchAndRewrite` can only erase the matched op or its descendants.
This is verified under expensive checks.
* It does not perform folding / DCE.
We could probably relax some of these in the future without sacrificing
too much performance.
