[mlir] Optimize usage of llvm::mapped_iterator

mapped_iterator is a useful abstraction for applying a
map function over an existing iterator, but our current
usage ends up allocating storage/making indirect calls
even with the map function is a known function, which
is horribly inefficient. This commit refactors the usage
of mapped_iterator to avoid this, and allows for directly
referencing the map function when dereferencing.

Fixes PR52319

Differential Revision: https://reviews.llvm.org/D113511