arm: Auto-vectorization for MVE: vsub

arm: Auto-vectorization for MVE: vsub

This patch enables MVE vsub instructions for auto-vectorization.
The sub<mode>3 in vec-common.md is modified to use new mode macros
to include MVE extension for vectorization. MVE vsub insns in mve.md are
modified to use 'minus' instead of unspec expression to support
sub<mode>3. Use VDQ instead fo VALL to cover all supported modes. The
redundant sub<mode>3 insns in neon.md are then removed.

gcc/ChangeLog:

2020-10-23  Dennis Zhang  <dennis.zhang@arm.com>

	* config/arm/mve.md (mve_vsubq<mode>): New entry for vsub instruction
	using expression 'minus'.
	(mve_vsubq_f<mode>): Use minus instead of VSUBQ_F unspec.
	* config/arm/neon.md (sub<mode>3, sub<mode>3_fp16): Removed.
	(neon_vsub<mode>): Use gen_sub<mode>3 instead of gen_sub<mode>3_fp16.
	* config/arm/vec-common.md (sub<mode>3): Use the new mode macros
	ARM_HAVE_<MODE>_ARITH. Use iterator VDQ instead of VALL.

gcc/testsuite/ChangeLog:

	* gcc.target/arm/simd/mve-vsub_1.c: New test.