compiler: buggy compose_nodes, drops HaloSpots

[georgebisbas]

buggy compose_nodes, drops HaloSpots to the floor thus leading to wrong number of collapsed loops

MFE:

from devito import configuration
configuration['language'] = 'C'
configuration['platform'] = 'bdw'  # Optimize for an Intel Broadwell
configuration['opt-options']['par-collapse-ncores'] = 1  # Maximize use loop collapsing
from sympy import sin
from devito import Eq, Grid, Operator, Function, TimeFunction
grid = Grid(shape=(80, 80, 80))
f = Function(name='f', grid=grid)
u = TimeFunction(name='u', grid=grid, space_order=4)
eq = Eq(u.forward, f**2*sin(f)*u.dy.dy)
op1_omp = Operator(eq, opt=('blocking', 'openmp'))

Code is using collapse(3) instead of collapse(1)

#1673 solves this issue