[AArch64] Factor out ptrue predicate creation

[AArch64] Factor out ptrue predicate creation

This is the first step to canonicalising predicate constants so that
they can be reused between modes.

2019-06-18  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64-protos.h (aarch64_ptrue_reg): Declare.
	* config/aarch64/aarch64.c (aarch64_ptrue_reg): New functions.
	(aarch64_expand_sve_widened_duplicate, aarch64_expand_sve_mem_move)
	(aarch64_maybe_expand_sve_subreg_move, aarch64_evpc_rev_local)
	(aarch64_expand_sve_vec_cmp_int): Use it.
	(aarch64_expand_sve_vec_cmp_float): Likewise.
	* config/aarch64/aarch64-sve.md: Likewise throughout.

From-SVN: r272424