nir/range_analysis: Make sure the table validation only occurs once

nir/range_analysis: Make sure the table validation only occurs once

All of the tables are static const, so they only need to be validated
once.  As noted in the previous commit, the compiler should be able to
eliminate all of this code when the assertions would pass.  Even with
the help of the previous commit, this does not always occur.

-Og: -95.688 +/- 3.91935 (-24.9562% +/- 1.0222%) N=5
-O1: No difference proven at 95.0% confidence. N=5
-O2: -1.962 +/- 0.85001 (-0.860013% +/- 0.372589%) N=5

Reviewed-by: Eric Anholt <eric@anholt.net>