This commit marks the type converter in `populate...` functions as
`const`. This is useful for debugging.
Patterns already take a `const` type converter. However, some
`populate...` functions do not only add new patterns, but also add
additional type conversion rules. That makes it difficult to find the
place where a type conversion was added in the code base. With this
change, all `populate...` functions that only populate pattern now have
a `const` type converter. Programmers can then conclude from the
function signature that these functions do not register any new type
conversion rules.
Also some minor cleanups around the 1:N dialect conversion
infrastructure, which did not always pass the type converter as a
`const` object internally.
MLIR can't really be const-correct (it would need a `ConstValue` class
alongside the `Value` class really, like `ArrayRef` and
`MutableArrayRef`). This is however making is more consistent: method
that are directly modifying the Value shouldn't be marked const.
Do not match on illegal destination blocks. Also apply some minor
cleanups.
TODO: We should add region/block argument type conversions to properly
address this.
Issue: https://github.com/llvm/llvm-project/issues/70813
