[mlir][Transforms] Dialect conversion: Simplify handling of dropped arguments (#96207)

This commit simplifies the handling of dropped arguments and updates
some dialect conversion documentation that is outdated.

When converting a block signature, a `BlockTypeConversionRewrite` object
and potentially multiple `ReplaceBlockArgRewrite` are created. During
the "commit" phase, uses of the old block arguments are replaced with
the new block arguments, but the old implementation was written in an
inconsistent way: some block arguments were replaced in
`BlockTypeConversionRewrite::commit` and some were replaced in
`ReplaceBlockArgRewrite::commit`. The new
`BlockTypeConversionRewrite::commit` implementation is much simpler and
no longer modifies any IR; that is done only in `ReplaceBlockArgRewrite`
now. The `ConvertedArgInfo` data structure is no longer needed.

To that end, materializations of dropped arguments are now built in
`applySignatureConversion` instead of `materializeLiveConversions`; the
latter function no longer has to deal with dropped arguments.

Other minor improvements:
- Improve variable name: `origOutputType` -> `origArgType`. Add an
assertion to check that this field is only used for argument
materializations.
- Add more comments to `applySignatureConversion`.

Note: Error messages around failed materializations for dropped basic
block arguments changed slightly. That is because those materializations
are now built in `legalizeUnresolvedMaterialization` instead of
`legalizeConvertedArgumentTypes`.

This commit is in preparation of decoupling argument/source/target
materializations from the dialect conversion.

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