[mlir][Transforms] Dialect Conversion: Add `replaceOpWithMultiple` (#115816)

This commit adds a new function
`ConversionPatternRewriter::replaceOpWithMultiple`. This function is
similar to `replaceOp`, but it accepts multiple `ValueRange`
replacements, one per op result.

Note: This function is not an overload of `replaceOp` because of
ambiguous overload resolution that would make the API difficult to use.

This commit aligns "block signature conversions" with "op replacements":
both support 1:N replacements now. Due to incomplete 1:N support in the
dialect conversion driver, an argument materialization is inserted when
an SSA value is replaced with multiple values; same as block signature
conversions already work around the problem. These argument
materializations are going to be removed in a subsequent commit that
adds full 1:N support. The purpose of this PR is to add missing features
gradually in small increments.

This commit also updates two MLIR transformations that have their custom
workarounds around missing 1:N support. These can already start using
`replaceOpWithMultiple`.

Co-authored-by: Markus Böck <markus.boeck02@gmail.com>

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