nir: Make some notes about fsign versus NaN

nir: Make some notes about fsign versus NaN

This commit only documents the current behavior, even if that behavior
is not the behavior preferred by the relevant specs.

In SPIR-V, there are two flavors of the sign instruction, and each lives
in an extended instruction set.  The GLSL.std.450 FSign instruction is
defined as:

    Result is 1.0 if x > 0, 0.0 if x = 0, or -1.0 if x < 0.

This also matches the GLSL 4.60 definition.

However, the OpenCL.ExtendedInstructionSet.100 sign instruction is
defined as:

    Returns 1.0 if x > 0, -0.0 if x = -0.0, +0.0 if x = +0.0, or -1.0 if
    x < 0. Returns 0.0 if x is a NaN.

There are two differences.  Each treats -0.0 differently, and each also
treats NaN differently.  Specifically, GLSL.std.450 FSign does not
define any specific behavior for NaN.

There has been some discussion in Khronos about the NaN behavior of
GLSL.std.450 FSign.  As part of that discussion, I did some research
into how we treat NaN for nir_op_fsign, and this commit just captures
some of those notes.

v2: Document the expected behavior of nir_op_fsign more thoroughly.
Suggested by Rhys.  Note that the current implementation of constant
folding does not produce the expected result for NaN.  Suggested by
Caio.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com> [v1]
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6358>