gallium/u_simple_shaders: Optimize out ffloors

gallium/u_simple_shaders: Optimize out ffloors

ffloor(f2i(x)) can't be optimized to f2i(x) due to differing behaviour for
negative x, but u_blitter only uses this with nonnegative x so we can instead
use ftrunc(f2i(x)) which NIR will optimize to f2i(x) for us. This gets rid of
the silly ffloor instructions in blit shaders.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24319>