[AArch64] Canonicalise SVE predicate constants

[AArch64] Canonicalise SVE predicate constants

This patch makes sure that we build all SVE predicate constants as
VNx16BI before RA, to encourage similar constants to be reused
between modes.  This is also useful for the ACLE, where the single
predicate type svbool_t is always a VNx16BI.

Also, and again to encourage reuse, the patch makes us use a .B PTRUE
for all ptrue-predicated operations, rather than (for example) using
a .S PTRUE for 32-bit operations and a .D PTRUE for 64-bit operations.

The only current case in which a .H, .S or .D operation needs to be
predicated by a "strict" .H/.S/.D PTRUE is the PTEST in a conditional
branch, which an earlier patch fixed to use an appropriate VNx16BI
constant.

2019-08-14  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_target_reg): New function.
	(aarch64_emit_set_immediate): Likewise.
	(aarch64_ptrue_reg): Build a VNx16BI constant and then bitcast it.
	(aarch64_pfalse_reg): Likewise.
	(aarch64_convert_sve_data_to_pred): New function.
	(aarch64_sve_move_pred_via_while): Take an optional target register
	and the required register mode.
	(aarch64_expand_sve_const_pred_1): New function.
	(aarch64_expand_sve_const_pred): Likewise.
	(aarch64_expand_mov_immediate): Build an all-true predicate
	if the significant bits of the immediate are all true.  Use
	aarch64_expand_sve_const_pred for all compile-time predicate constants.
	(aarch64_mov_operand_p): Force predicate constants to be VNx16BI
	before register allocation.
	* config/aarch64/aarch64-sve.md (*vec_duplicate<mode>_reg): Use
	a VNx16BI PTRUE when splitting the memory alternative.
	(vec_duplicate<mode>): Update accordingly.
	(*pred_cmp<cmp_op><mode>): Rename to...
	(@aarch64_pred_cmp<cmp_op><mode>): ...this.

gcc/testsuite/
	* gcc.target/aarch64/sve/spill_4.c: Expect all ptrues to be .Bs.
	* gcc.target/aarch64/sve/single_1.c: Likewise.
	* gcc.target/aarch64/sve/single_2.c: Likewise.
	* gcc.target/aarch64/sve/single_3.c: Likewise.
	* gcc.target/aarch64/sve/single_4.c: Likewise.

From-SVN: r274415