Add an emitc.for op to the EmitC dialect as a lowering target for
scf.for, replacing its current direct translation to C; The translator
now handles emitc.for instead.
This patch recommits 126f0374cbc2110aa97e2141ac898014a8b9531a, reverted by
3ada774d0f65b44f21b360d222f446e533df1a34, along with the missing dependence.
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.
