Add an eager API version for BR and COND_BR

When building unstructured control-flow there is a need to construct mlir::Block* before being able to fill them. This invites goto-style programming.

This CL introduces an alternative eager API for BR and COND_BR in which blocks are created eagerly and captured on the fly.

This allows reducing the number of calls to `BlockBuilder` from 4 to 2 in the `builder_blocks_eager` test and from 3 to 2 in the `builder_cond_branch_eager` test.

PiperOrigin-RevId: 237046114