Always emit weight expression.

[thorstenhater]

This addresses a problem occuring when a ionic (accumulated) quantity is
declared as WRITE, we have a block (any will do) that just reads it and
modcc emits SIMD code.

Example

NEURON {
  USEION na WRITE nai
}

INITIAL {
   LOCAL foo
   foo = nai
} << BOOM

BREAKPOINT {
  nai = 42
}

In this case modcc conservatively assumes that it will be written as well and emits

t_nai = S::fma(w_, nai, t_nai);

but 'forgets' to define w_. In scalar mode it's fine, since we just use weight[index]
without a temporary.

Solution

Be less fancy, just emit the SIMD code for

 w_ = weight[index];

equally conservatively, read always.