st/mesa: fix up result_src.type when doing i2u/u2i conversions

st/mesa: fix up result_src.type when doing i2u/u2i conversions

Even though it's a no-op, it's important to keep track of the type so
that we can pick the properly-signed op later on.

This fixes dEQP-GLES3.functional.shaders.precision.uint.highp_div_fragment,
which ended up using IDIV instead of UDIV.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: mesa-stable@lists.freedesktop.org