arm: Auto-vectorization for MVE: vneg

This patch enables MVE vneg instructions for auto-vectorization.  MVE
vnegq insns in mve.md are modified to use 'neg' instead of unspec
expression.  The neg<mode>2 expander is added to vec-common.md.

Existing patterns in neon.md are prefixed with neon_.
It's not clear why we have different patterns for VDQW
and VH in neon.md, when WDQWH handles both, and patterns
with VDQ have provision for attributes for FP modes.

Another question is why <absneg_str><mode>2 always sets
neon_abs<q> type when it also handles neon_neq<q> cases.

2020-12-11  Christophe Lyon  <christophe.lyon@linaro.org>

	gcc/
	* config/arm/mve.md (mve_vnegq_f): Use 'neg' instead of unspec.
	(mve_vnegq_s): Likewise.
	* config/arm/neon.md (neg<mode>2): Rename into neon_neg<mode>2.
	(<absneg_str><mode>2): Rename into neon_<absneg_str><mode>2.
	(neon_v<absneg_str><mode>): Call gen_neon_<absneg_str><mode>2.
	(vashr<mode>3): Call gen_neon_neg<mode>2.
	(vlshr<mode>3): Call gen_neon_neg<mode>2.
	(neon_vneg<mode>): Call gen_neon_neg<mode>2.
	* config/arm/unspecs.md (VNEGQ_F, VNEGQ_S): Remove.
	* config/arm/vec-common.md (neg<mode>2): New expander.

	gcc/testsuite/
	* gcc.target/arm/simd/mve-vneg.c: Add tests for vneg.