add_insn_allocno_copies and its subroutines used HARD_REG_SET to
represent a bitmask of alternatives.  There's not really any connection
between the number of registers and the maximum number of alternatives,
so this patch uses alternative_mask instead (which wasn't around when
this code was added).

This is just a minor clean-up making way for later patches.

2019-07-01  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* ira-int.h (ira_setup_alts, ira_get_dup_out_num): Use
	alternative_mask instead of HARD_REG_SET to represent a
	bitmask of alternatives.
	* ira.c (ira_setup_alts, ira_get_dup_out_num): Likewise.
	* ira-conflicts.c (add_insn_allocno_copies): Likewise.

From-SVN: r272847