nv50/ir: Initialize ValueDef member origin in constructors.

Fix defects reported by Coverity Scan.

Uninitialized pointer field (UNINIT_CTOR)
uninit_member: Non-static class member origin is not initialized in this
constructor nor in any functions that it calls.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9036>

src/gallium/drivers/nouveau/codegen/nv50_ir.cpp

@@ -111,12 +111,12 @@ bool ValueRef::getImmediate(ImmediateValue &imm) const
    return false;
 }
 
-ValueDef::ValueDef(Value *v) : value(NULL), insn(NULL)
+ValueDef::ValueDef(Value *v) : value(NULL), origin(NULL), insn(NULL)
 {
    set(v);
 }
 
-ValueDef::ValueDef(const ValueDef& def) : value(NULL), insn(NULL)
+ValueDef::ValueDef(const ValueDef& def) : value(NULL), origin(NULL), insn(NULL)
 {
    set(def.get());
 }