aco: Initialize union within Operand for MSVC

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7785>

src/amd/compiler/aco_ir.h

@@ -770,10 +770,10 @@ public:
 
 private:
    union {
+      Temp temp;
       uint32_t i;
       float f;
-      Temp temp = Temp(0, s1);
+   } data_ = { Temp(0, s1) };
-   } data_;
    PhysReg reg_;
    union {
       struct {