nir/range-analysis: Add pragmas to help loop unrolling

nir/range-analysis: Add pragmas to help loop unrolling

I was pretty liberal with these assertions when I wrote this code
because I had assumed that GCC would unroll the loops, inline the look ups
of static const arrays with now constant indices, and then elmininate
all the actuall assertions.  It seems none of this happens even at -O3.

Adding the pragmas helps encourage loop unrolling at some optimization
levels.  I tested by running shader-db with NIR_VALIDATE=false on a Core
i7 Haswell desktop system.

-Og: No difference proven at 95.0% confidence. N=5
-O1: -48.304 +/- 1.221 (-16.3343% +/- 0.412888%) N=5
-O2: -49.94 +/- 1.23521 (-17.9634% +/- 0.444303%) N=5

v2: Add a _Pragma to an inner loop that was accidentally dropped during
a rebase.

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