This patch adds helper functions that say which of the two modes
involved in a subreg is the larger, preferring the outer mode in
the event of a tie.  It also converts IRA and reload to track modes
instead of byte sizes, since this is slightly more convenient when
variable-sized modes are added later.

2017-10-26  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* rtl.h (wider_subreg_mode): New function.
	* ira.h (ira_sort_regnos_for_alter_reg): Take a machine_mode *
	rather than an unsigned int *.
	* ira-color.c (regno_max_ref_width): Replace with...
	(regno_max_ref_mode): ...this new variable.
	(coalesced_pseudo_reg_slot_compare): Update accordingly.
	Use wider_subreg_mode.
	(ira_sort_regnos_for_alter_reg): Likewise.  Take a machine_mode *
	rather than an unsigned int *.
	* lra-constraints.c (uses_hard_regs_p): Use wider_subreg_mode.
	(process_alt_operands): Likewise.
	(invariant_p): Likewise.
	* lra-spills.c (assign_mem_slot): Likewise.
	(add_pseudo_to_slot): Likewise.
	* lra.c (collect_non_operand_hard_regs): Likewise.
	(add_regs_to_insn_regno_info): Likewise.
	* reload1.c (regno_max_ref_width): Replace with...
	(regno_max_ref_mode): ...this new variable.
	(reload): Update accordingly.  Update call to
	ira_sort_regnos_for_alter_reg.
	(alter_reg): Update to use regno_max_ref_mode.  Call wider_subreg_mode.
	(init_eliminable_invariants): Update to use regno_max_ref_mode.
	(scan_paradoxical_subregs): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r254115