mesa: Dynamically allocate acp array in ir_to_mesa_visitor::copy_propagate.

mesa: Dynamically allocate acp array in ir_to_mesa_visitor::copy_propagate.

Fixes these MSVC errors.
ir_to_mesa.cpp(2644) : error C2057: expected constant expression
ir_to_mesa.cpp(2644) : error C2466: cannot allocate an array of constant size 0
ir_to_mesa.cpp(2644) : error C2133: 'acp' : unknown size
ir_to_mesa.cpp(2646) : error C2070: 'ir_to_mesa_instruction *[]': illegal sizeof operand
ir_to_mesa.cpp(2709) : error C2070: 'ir_to_mesa_instruction *[]': illegal sizeof operand
ir_to_mesa.cpp(2718) : error C2070: 'ir_to_mesa_instruction *[]': illegal sizeof operand