Fixed addition of SOLVE block to kernel's FOR loop

[georgemitenkov]

This is a small PR to fix append_statements_from_block function in LLVM helper visitor. Before, if nonspecific current was not specified, the whole BREAKPOINT block would be added to the kernel body. This led to cases when SOLVE block was together with the actual solution to DERIVATIVE:

VOID nrn_state_test(INSTANCE_STRUCT *mech){
    INTEGER id
    INTEGER node_id
    DOUBLE v
    for(id = 0; id<mech->node_count-1; id = id+2) {
        node_id = mech->node_index[id]
        v = mech->voltage[node_id]
        mech->m[id] = mech->m[id]+(1.0-exp(mech->dt*(1.0)))*(-(0.0)/(1.0)-mech->m[id])
        SOLVE states METHOD cnexp
        // Rest of BREAKPOINT code
    }
    // Rest of kernel
}

Now, a statement expression is checked before appending so that SOLVE block is not added.

[bbpbuildbot]

Can one of the admins verify this patch?

[pramodk]

LGTM - thanks for the catch!