Add TARGET_ADDR_SPACE_ZERO_ADDRESS_VALID

Add TARGET_ADDR_SPACE_ZERO_ADDRESS_VALID

        * gimple.c (check_loadstore): Return false when 0 is a valid address.
        * fold-const.c (const_unop) [ADDR_SPACE_CONVERT_EXPR]: Do not fold
        null when 0 is valid in the source address space.
        * target.def (TARGET_ADDR_SPACE_ZERO_ADDRESS_VALID): New.
        * targhooks.c (default_addr_space_zero_address_valid): New.
        * targhooks.h (default_addr_space_zero_address_valid): Declare.
        * doc/tm.texi.in (TARGET_ADDR_SPACE_ZERO_ADDRESS_VALID): Mark it.
        * doc/tm.texi: Rebuild.

From-SVN: r229999