nouveau/codegen: set dType to S32 for OP_NEG U32

nouveau/codegen: set dType to S32 for OP_NEG U32

It doesn't make sense to do an OP_NEG from U32 to U32. This was
manifested on nv50 in glsl-fs-atan-3 which was generating a

UMAD TEMP[0].x, TEMP[0].xxxx, -TEMP[5].xxxx, TEMP[0].xxxx

instruction. (For some reason, nvc0 causes a different shader to be
generated.) This led to a

cvt neg u32 $r1 u32 $r1

Which did not yield the desired result. This changes the final output to

cvt neg s32 $r1 u32 $r1

which produces the desired output and the piglit tests passes. My
assumption is that this is also what we want on nvc0, but could not test
as there was no suitable shader that generated the problem instruction.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>