This is kind of a hack.  We currently track precise requirements
by decorating ir_variables.  Propagating or grafting the RHS of an
assignment to a precise value into some other expression tree can
lose those decorations.

In the long run, it might be better to replace these ir_variable
decorations with an "exact" decoration on ir_expression nodes,
similar to what NIR does.

In the short run, this is probably good enough.  It preserves
enough information for glsl_to_nir to generate "exact" decorations,
and NIR will then handle optimizing these expressions reasonably.

Fixes ES31-CTS.gpu_shader5.precise_qualifier.

v2: Drop invariant handling, as it shouldn't be necessary (caught
    by Jason Ekstrand).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>