nir: Implement optional b2f->iand lowering

This pass is required by the Midgard compiler; our instruction set uses
NIR-style booleans (~0 for true) but lacks a dedicated b2f instruction.
Normally, this lowering pass would be implemented in a backend-specific
algebraic pass, but this conflicts with the existing iand->b2f pass in
nir_opt_algebraic.py, hanging the compiler. This patch thus makes the
existing pass optional (default on -- all other backends should remain
unaffected), adding an optional pass for lowering the opposite
direction.

v2: Defer lowering until late algebraic optimisations to allow
optimising the b2f instruction itself.

Signed-off-by: Alyssa Rosenzweig <[email protected]>
Reviewed-by: Matt Turner <[email protected]>
Reviewed-by: Christian Gmeiner <[email protected]>

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