gallivm: fix float->SNORM conversion

gallivm: fix float->SNORM conversion

Just like the UNORM case we need to use round to nearest, not trunc.
(There's also another problem, we're using the formula for SNORM->float
which will produce a value below -1.0 for the most negative value which
according to both OpenGL and d3d10 would need clamping. However, no actual
failures have been observed due to that hence keep cheating on that.)

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>