nir/algebraic: Add a bit-size validator

nir/algebraic: Add a bit-size validator

This commit adds a validator that ensures that all expressions passed
through nir_algebraic are 100% non-ambiguous as far as bit-sizes are
concerned.  This way it's a compile-time error rather than a hard-to-trace
C exception some time later.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>