[AArch64] Add gather loads for partial SVE modes

[AArch64] Add gather loads for partial SVE modes

This patch adds support for gather loads of partial vectors,
where the vector base or offset elements can be wider than the
elements being loaded.

2019-11-16  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/iterators.md (SVE_24, SVE_2, SVE_4): New mode
	iterators.
	* config/aarch64/aarch64-sve.md
	(gather_load<SVE_FULL_SD:mode><v_int_equiv>): Extend to...
	(gather_load<SVE_24:mode><v_int_container>): ...this.
	(mask_gather_load<SVE_FULL_S:mode><v_int_equiv>): Extend to...
	(mask_gather_load<SVE_4:mode><v_int_container>): ...this.
	(mask_gather_load<SVE_FULL_D:mode><v_int_equiv>): Extend to...
	(mask_gather_load<SVE_2:mode><v_int_container>): ...this.
	(*mask_gather_load<SVE_2:mode><v_int_container>_<su>xtw_unpacked):
	New pattern.
	(*mask_gather_load<SVE_FULL_D:mode><v_int_equiv>_sxtw): Extend to...
	(*mask_gather_load<SVE_2:mode><v_int_equiv>_sxtw): ...this.
	Allow the nominal extension predicate to be different from the
	load predicate.
	(*mask_gather_load<SVE_FULL_D:mode><v_int_equiv>_uxtw): Extend to...
	(*mask_gather_load<SVE_2:mode><v_int_equiv>_uxtw): ...this.

gcc/testsuite/
	* gcc.target/aarch64/sve/gather_load_1.c (TEST_LOOP): Start at 0.
	(TEST_ALL): Add tests for 8-bit and 16-bit elements.
	* gcc.target/aarch64/sve/gather_load_2.c: Update accordingly.
	* gcc.target/aarch64/sve/gather_load_3.c (TEST_LOOP): Start at 0.
	(TEST_ALL): Add tests for 8-bit and 16-bit elements.
	* gcc.target/aarch64/sve/gather_load_4.c: Update accordingly.
	* gcc.target/aarch64/sve/gather_load_5.c (TEST_LOOP): Start at 0.
	(TEST_ALL): Add tests for 8-bit, 16-bit and 32-bit elements.
	* gcc.target/aarch64/sve/gather_load_6.c: Add
	--param aarch64-sve-compare-costs=0.
	(TEST_LOOP): Start at 0.
	* gcc.target/aarch64/sve/gather_load_7.c: Add
	--param aarch64-sve-compare-costs=0.
	* gcc.target/aarch64/sve/gather_load_8.c: New test.
	* gcc.target/aarch64/sve/gather_load_9.c: Likewise.
	* gcc.target/aarch64/sve/mask_gather_load_6.c: Add
	--param aarch64-sve-compare-costs=0.

From-SVN: r278345