[mlir][NFC] Refactor DialectConversion debug logging

Summary:
This revision beefs up the debug logging within dialect conversion. Given the nature of multi-level legalization, and legalization in general, it is one of the harder pieces of infrastructure to debug. This revision adds nice formatting to make the output log easier to parse:

```
Legalizing operation : 'std.constant'(0x608000002420) {
  * Fold {
  } -> FAILURE : unable to fold

  * Pattern : 'std.constant -> ()' {
  } -> FAILURE : pattern failed to match

  * Pattern : 'std.constant -> ()' {
  } -> FAILURE : pattern failed to match

  * Pattern : 'std.constant -> (spv.constant)' {
    ** Insert  : 'spv.constant'(0x608000002c20)
    ** Replace : 'std.constant'(0x608000002420)

    //===-------------------------------------------===//
    Legalizing operation : 'spv.constant'(0x608000002c20) {
    } -> SUCCESS : operation marked legal by the target
    //===-------------------------------------------===//
  } -> SUCCESS : pattern applied successfully
} -> SUCCESS
```

Differential Revision: https://reviews.llvm.org/D73747