[mlir][func] Replace `ValueDecomposer` with target materialization (#114192)

The `ValueDecomposer` in `DecomposeCallGraphTypes` was a workaround
around missing 1:N support in the dialect conversion. Since #113032, the
dialect conversion infrastructure supports 1:N type conversions and 1:N
target materializations. The `ValueDecomposer` class is no longer
needed. (However, target materializations must still be inserted
manually, until we fully merge the 1:1 and 1:N drivers.)

Note for LLVM integration: Register 1:N target materializations on the
type converter instead of "decompose value conversions" on the
`ValueDecomposer`.