Use block walk for ssa_cons_start

Use block walk for ssa_cons_start

The node walk used before was inefficient,
since everything but blocks was ignored.

Additionally, this fixes a bug, because ssa_cons_finish already used a
block walk already, such that unreachable blocks were immatured,
but not matured in the scalar-replace optimization. This bug could
also be avoided by ensuring cfopt before scalar-replace, but keeping
ssacons start and finish symmetric seems to be more robust.