EDSC: support conditional branch instructions

Leverage the recently introduced support for multiple argument groups and
multiple destination blocks in EDSC Expressions to implement conditional
branches in EDSC.  Conditional branches have two successors and three argument
groups.  The first group contains a single expression of i1 type that
corresponds to the condition of the branch.  The two following groups contain
arguments of the two successors of the conditional branch instruction, in the
same order as the successors.  Expose this instruction to the C API and Python
bindings.

PiperOrigin-RevId: 235542768