ReplaceBuilder is available in the public API through
`DataFlowGraph::replace`, however it's documentation is not available
through rustdoc as the type isn't publicly importable.
This move allows the `IntCC`/`FloatCC` enums to be used in both meta (for predicate matching) and in codegen. To avoid breaking any code dependent on the previous location of condcodes.rs (`cranelift-codegen/src/condcodes.rs`), the module is re-exported under `cranelift_codegen::ir`.
