nv/codegen: Change copy-constructor call to assign

This almost certainly intends to call the user-definied assignment
operator here instead of the automatically generated copy constructor.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24904>

src/nouveau/codegen/nv50_ir_peephole.cpp

@@ -744,7 +744,8 @@ ConstantFolding::expr(Instruction *i,
    switch (i->op) {
    case OP_MAD:
    case OP_FMA: {
-      ImmediateValue src0, src1 = *i->getSrc(0)->asImm();
+      ImmediateValue src0, src1;
+      src1 = *i->getSrc(0)->asImm();
 
       // Move the immediate into position 1, where we know it might be
       // emittable. However it might not be anyways, as there may be other