re PR rtl-optimization/50339 (suboptimal register allocation for abs(__int128_t))

	PR rtl-optimization/50339
	* lower-subreg.h (struct lower_subreg_choices): Add splitting_ashiftrt
	field.
	* lower-subreg.c (compute_splitting_shift): Handle ASHIFTRT.
	(compute_costs): Call compute_splitting_shift also for ASHIFTRT
	into splitting_ashiftrt field.
	(find_decomposable_shift_zext, resolve_shift_zext): Handle also
	ASHIFTRT.
	(dump_choices): Fix up printing LSHIFTRT choices, print ASHIFTRT
	choices.

From-SVN: r196214