Add a structured if operation (#67234)

Add an emitc.if op to the EmitC dialect. A new convert-scf-to-emitc
pass replaces the existing direct translation of scf.if to C; The
translator now handles emitc.if instead.

The emitc.if op doesn't return any value and its then/else regions are
terminated with a new scf.yield op. Values returned by scf.if are
lowered using emitc.variable ops, assigned to in the then/else regions
using a new emitc.assign op.