m68k: Accept ASHIFT like MULT in address operand

When LRA pulls an address operand out of a MEM it caninoicalizes a
containing MULT into ASHIFT.  Adjust the address decomposer to recognize
this form.

	PR target/116413
	* config/m68k/m68k.cc (m68k_decompose_index): Accept ASHIFT like
	MULT.
	(m68k_rtx_costs) [PLUS]: Likewise.
	(m68k_legitimize_address): Likewise.