target-insns.def: (compose_tag) New pattern.

target-insns.def: (compose_tag) New pattern.

Add a new target instruction used by hardware-assisted sanitizers on
architectures providing memory-tagging instructions. This instruction
is used to compute assign tags at a fixed offset from a tagged address
base. For example, in AArch64 case, this pattern instantiate `addg`
instruction.

	gcc/
	* target-insns.def (compose_tag): New target instruction.
	* doc/md.texi (compose_tag): Add documentation.

Signed-off-by: Claudiu Zissulescu <claudiu.zissulescu-ianculescu@oracle.com>