nir/algebraic: mark float optimizations returning one parameter as inexact

nir/algebraic: mark float optimizations returning one parameter as inexact

With the arrival of VK_KHR_shader_float_controls algebraic
optimizations for float types of the form (('fop', a, b), a) become
inexact depending on the execution mode.

For example, if we have activated SHADER_DENORM_FLUSH_TO_ZERO, in case
of a denorm value for the "a" parameter, we cannot return it still as
a denorm, it needs to be flushed to zero. Therefore, we mark now all
those operations as inexact.

Suggested-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>