aarch64: Enable aarch64_load to use UNSPEC_PRED_X loads

This patch will enable the use of loads using the UNSPEC_PRED_X enum in the
aarch64_load pattern, thus enabling combine to combine such loads with extends.

gcc/ChangeLog:
2021-05-19  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	* config/aarch64/iterators.md (SVE_PRED_LOAD): New iterator.
	(pred_load): New int attribute.
	* config/aarch64/aarch64-sve.md
	(aarch64_load_<ANY_EXTEND:optab><SVE_HSDI:mode><SVE_PARTIAL_I:mode>): Use
	SVE_PRED_LOAD enum iterator and corresponding pred_load attribute.
	* config/aarch64/aarch64-sve-builtins-base.cc (expand): Update call to
	code_for_aarch64_load.

gcc/testsuite/ChangeLog:
2021-05-19  Andre Vieira  <andre.simoesdiasvieira@arm.com>

	* gcc.target/aarch64/sve/logical_unpacked_and_2.c: Change
	scan-assembly-times to scan-assembly not for superfluous uxtb.
	* gcc.target/aarch64/sve/logical_unpacked_and_3.c: Likewise.
	* gcc.target/aarch64/sve/logical_unpacked_and_4.c: Likewise.
	* gcc.target/aarch64/sve/logical_unpacked_and_6.c: Likewise.
	* gcc.target/aarch64/sve/logical_unpacked_and_7.c: Likewise.
	* gcc.target/aarch64/sve/logical_unpacked_eor_2.c: Likewise.
	* gcc.target/aarch64/sve/logical_unpacked_eor_3.c: Likewise.
	* gcc.target/aarch64/sve/logical_unpacked_eor_4.c: Likewise.
	* gcc.target/aarch64/sve/logical_unpacked_eor_6.c: Likewise.
	* gcc.target/aarch64/sve/logical_unpacked_eor_7.c: Likewise.
	* gcc.target/aarch64/sve/logical_unpacked_orr_2.c: Likewise.
	* gcc.target/aarch64/sve/logical_unpacked_orr_3.c: Likewise.
	* gcc.target/aarch64/sve/logical_unpacked_orr_4.c: Likewise.
	* gcc.target/aarch64/sve/logical_unpacked_orr_6.c: Likewise.
	* gcc.target/aarch64/sve/logical_unpacked_orr_7.c: Likewise.
	* gcc.target/aarch64/sve/ld1_extend.c: New test.