llvmpipe: Use saturating add/sub for UNORM formats

lp_build_add and lp_build_sub have fallback code for cases
that cannot be handled by known intrinsics.  For UNORM formats,
this code was using modulo rather than saturating arithmetic.

This fixes some rendering issues for a gnome session on System z.
It also fixes various piglit tests on z, such as
spec/ARB_color_buffer_float/GL_RGBA8-render.

The patch deliberately doesn't tackle the more complicated
SNORM case.

Tested against piglit on x86_64 and System z with no regressions.

Reviewed-by: Adam Jackson <[email protected]>
Signed-off-by: Richard Sandiford <[email protected]>

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