[mlir][Transforms][NFC] Dialect Conversion: Resolve insertion point TODO (#95653)

Remove a TODO in the dialect conversion code base when materializing
unresolved conversions:
```
// FIXME: Determine a suitable insertion location when there are multiple
// inputs.
```

The implementation used to select an insertion point as follows:
- If the cast has exactly one operand: right after the definition of the
SSA value.
- Otherwise: right before the cast op.

However, it is not necessary to change the insertion point. Unresolved
materializations (`UnrealizedConversionCastOp`) are built during
`buildUnresolvedArgumentMaterialization` or
`buildUnresolvedTargetMaterialization`. In the former case, the op is
inserted at the beginning of the block. In the latter case, only one
operand is supported in the dialect conversion, and the op is inserted
right after the definition of the SSA value. I.e., the
`UnrealizedConversionCastOp` is already inserted at the right place and
it is not necessary to change the insertion point for the resolved
materialization op.

Note: The IR change changes slightly because the
`unrealized_conversion_cast` ops at the beginning of a block are no
longer doubly-inverted (by setting the insertion to the beginning of the
block when inserting the `unrealized_conversion_cast` and again when
inserting the resolved conversion op). All affected test cases were
fixed by using `CHECK-DAG` instead of `CHECK`.

Also improve the quality of multiple test cases that did not check for
the correct operands.

Note: This commit is in preparation of decoupling the
argument/source/target materialization logic of the type converter from
the dialect conversion (to reduce its complexity and make that
functionality usable from a new dialect conversion driver).

0 files changed, 0 insertions, 0 deletions