nv50/ir: only take abs value when computing high result

Not reachable from TGSI since it only has UMUL, no IMUL. However it's
surprising that setting argument types to s32 will cause sign to get
lost.

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

src/gallium/drivers/nouveau/codegen/nv50_ir_lowering_nv50.cpp

@@ -75,7 +75,7 @@ expandIntegerMUL(BuildUtil *bld, Instruction *mul)
    s[0] = mul->getSrc(0);
    s[1] = mul->getSrc(1);
 
-   if (isSignedType(mul->sType)) {
+   if (isSignedType(mul->sType) && highResult) {
       s[0] = bld->getSSA(fullSize);
       s[1] = bld->getSSA(fullSize);
       bld->mkOp1(OP_ABS, mul->sType, s[0], mul->getSrc(0));