aco: always end linear vgprs at top-level blocks

This fixes a previously unconsidered case with p_end_linear_vgpr placement:

v0 = start_linear_vgpr
if (...) {

} else {
   use_linear_vgpr(v0)
}
v0 = phi

We need a p_end_linear_vgpr to ensure that the phi does not use the same VGPR as the linear VGPR.