[llvm] [mlir][Transform] Create a transform interpreter and a preloader pass (PR #68661)

[Nicolas Vasilache via llvm-commits]

================
@@ -32,8 +32,17 @@ transform.named_sequence @lower_to_cpu(
     partial_conversion
   } : !transform.any_op
 
-  %m2 = transform.apply_registered_pass "reconcile-unrealized-casts" to %module : (!transform.any_op) -> !transform.any_op
-  transform.yield %m2 : !transform.any_op
+  // Need to rematch here because:
+  //   1. applying reconcile-unrealized-casts on the whole module yields the
+  //      transform applies to transform, when called from a named sequence, at
+  //      this time.
+  //   2. apply_conversion patterns consumes the func but does not produce 
+  //      a new llvm.func.
+  %f6 = transform.structured.match ops{["llvm.func"]} in %module 
+    : (!transform.any_op) -> !transform.any_op
+  %f7 = transform.apply_registered_pass "reconcile-unrealized-casts" to %f6
+    : (!transform.any_op) -> !transform.any_op
+  transform.yield %module : !transform.any_op
----------------
nicolasvasilache wrote:

There are 2 things here:
1. some missing tracking that this PR exposes and that I was not able to find yet, I'll file a bug on top to get back to running reconcile-unrealized-casts on module. I am not completely sure how this interacts right now with point 2 below.
2. the other issue is that by dropping `{transform.target_tag="payload"}` support, we end up trying to apply transform to the transform module embedded along with the payload IR. This currently fails and should be fixed separately I believe.

https://github.com/llvm/llvm-project/pull/68661