Extend 4195a9450b so that the next poor fool doesn't come along and
say, "sge does the right thing for 16-bit sources, but slt gives a NIR
validation failure. What the deuce?"
NOTE: This commit is necessary to prevent regressions in GLSLstd450Step
tests of 16-bit sources at "spriv: Produce correct result for
GLSLstd450Step with NaN".
Fixes: 4195a9450b ("nir: sge operation is defined for floating-point types")
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13999>
38800b385c