🦑 Transform ASSIGNED to LOCAL

[thorstenhater]

We can safely transform an ASSIGNED var into a LOCAL iff the following condition holds for all API blocks
in an NMODL mechanism:var written before it's read. If var is never read, we can remove it alltogether.

The only wrinkle are if-then-else statements, but these are fine with a bit of care.
For that consider both arms (if present!) individually and merge their reads and
writes:

• if 'written-before-read' is true for both branches, then it's true overall
• if 'written-before-read' is true for one branch, dismiss and proceed
• if it's false for one branch, it's false overall
• if var is written in one branch, dismiss this fact
• if var is written in both branches, propagate that fact.

We miss some cases

ASSIGNED { q10 }

INITIAL { q10 = ...}

BREAKPOINT { ix = f(q10) }

but there it's questionable (without a cost model depending on the backend CPU/SIMD/GPU) whether it's
an optimisation anyhow. These could be recovered by ordering the blocks (and their possible interleavings,
potentially) by the order prescribed by fvm_lowered_cell::integrate, but I consider this quite brittle.

[thorstenhater]

Note

This also requires at least a primitive cost model as things that are expensive to compute and never change
are worth writing to memory. That also might depend on CPU ./. GPU.