[AArch64] Add truncation for partial SVE modes

[AArch64] Add truncation for partial SVE modes

This patch adds support for "truncating" to a partial SVE vector from
either a full SVE vector or a wider partial vector.  This truncation is
actually a no-op and so should have zero cost in the vector cost model.

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

gcc/
	* config/aarch64/aarch64-sve.md
	(trunc<SVE_HSDI:mode><SVE_PARTIAL_I:mode>2): New pattern.
	* config/aarch64/aarch64.c (aarch64_integer_truncation_p): New
	function.
	(aarch64_sve_adjust_stmt_cost): Call it.

gcc/testsuite/
	* gcc.target/aarch64/sve/mask_struct_load_1.c: Add
	--param aarch64-sve-compare-costs=0.
	* gcc.target/aarch64/sve/mask_struct_load_2.c: Likewise.
	* gcc.target/aarch64/sve/mask_struct_load_3.c: Likewise.
	* gcc.target/aarch64/sve/mask_struct_load_4.c: Likewise.
	* gcc.target/aarch64/sve/mask_struct_load_5.c: Likewise.
	* gcc.target/aarch64/sve/pack_1.c: Likewise.
	* gcc.target/aarch64/sve/truncate_1.c: New test.

From-SVN: r278344