[MIPS] Update the ZC constraint.

gcc/

	* config/mips/constraints.md (ZC): Add support for R6 LL/SC
	offsets.
	(ZD): Update to use ISA_HAS_9BIT_DISPLACEMENT.
	* config/mips/mips.h (ISA_HAS_PREFETCH_9BIT): Rename to...
	(ISA_HAS_9BIT_DISPLACEMENT): ... this. New macro.
	* config/mips/sync.md (sync_compare_and_swap<mode>): Use ZC
	instead of ZR for the memory operand of LL/SC.
	(compare_and_swap_12, sync_add<mode>): Likewise.
	(sync_<optab>_12, sync_old_<optab>_12): Likewise.
	(sync_new_<optab>_12, sync_nand_12): Likewise.
	(sync_old_nand_12, sync_new_nand_12): Likewise.
	(sync_sub<mode>, sync_old_add<mode>): Likewise.
	(sync_old_sub<mode>, sync_new_add<mode>): Likewise.
	(sync_new_sub<mode>, sync_<optab><mode>): Likewise.
	(sync_old_<optab><mode>, sync_new_<optab><mode>"): Likewise.
	(sync_nand<mode>, sync_old_nand<mode>): Likewise.
	(sync_new_nand<mode>, sync_lock_test_and_set<mode>): Likewise.
	(test_and_set_12, atomic_compare_and_swap<mode>): Likewise.
	(atomic_exchange<mode>_llsc, atomic_fetch_add<mode>_llsc): Likewise.
	* doc/md.texi (ZC): Update description.

From-SVN: r219619