nir/algebraic: Mark other comparison exact when removing a == a

nir/algebraic: Mark other comparison exact when removing a == a

This prevents some additional optimizations that would change the
original result.  This includes things like (b < a && b < c) => b <
min(a, c) and !(a < b) => b >= a.  Both of these optimizations were
specifically observed in the piglit tests added in piglit!160.

This was discovered while investigating
https://gitlab.freedesktop.org/mesa/mesa/issues/1958.  However, the
problem in that issue was Chrome or Angle is replacing calls to isnan()
with some stuff that we (correctly) optimize to false.  If they had left
the calls to isnan() alone, everything would have just worked.

No shader-db changes on any Intel platform.

I also tried marking the comparison generated by the isnan() function
precise.  The precise marker "infects" every computation involved in
calculating the parameter to the isnan() function, and this severely
hurt all of the (few) shaders in shader-db that use isnan().

I also considered adding a new ir_unop_isnan opcode that would implement
the functionality.  During GLSL IR-to-NIR translation, the resulting
comparison operation would be marked exact (and the samething would need
to happen in SPIR-V translation).

This approach taken by this patch seemed easier, but we may want to do
the ir_unop_isnan thing anyway.

Fixes: d55835b8bdf ("nir/algebraic: Add optimizations for "a == a && a CMP b"")
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>